[RFA/RFC] Stack clash mitigation patch 01/08 - V3

On Mon, Jul 31, 2017 at 7:35 AM, Jeff Law <law@redhat.com> wrote:
> This patch introduces the stack clash protection options
>
> Changes since V2:
>
> Adds two new params.  The first controls the size of the guard area.
> This controls the threshold for when a function prologue requires
> probes.  The second controls the probing interval -- ie, once probes are
> needed, how often do we emit them.  These are really meant more for
> developers to experiment with than users.  Regardless I did go ahead and
> document them./PARAM
>
> It also adds some sanity checking WRT combining stack clash protection
> with -fstack-check.


I don't like this.  Either use them as if they were power-of-two
(floor_log2/ceil_log2 as appropriate) or simply make them take
the logarithm instead (like -mincoming-stack-boundary and friends).

Both -fstack-clash-protection and -fstack-check cannot be turned
off per function.  This means they would need merging in lto-wrapper.
The alternative is to mark them with 'Optimization' and allow per-function
specification (like we do for -fstack-protector).

Otherwise ok.

Richard.

> Jeff
>
>
>         * common.opt (-fstack-clash-protection): New option.
>         * flag-types.h (enum stack_check_type): Note difference between
>         -fstack-check= and -fstack-clash-protection.
>         * params.h (set_param_value): Verify stack-clash-protection
>         params are powers of two.
>         * params.def (PARAM_STACK_CLASH_PROTECTION_GUARD_SIZE): New PARAM.
>         (PARAM_STACK_CLASH_PROTECTION_PROBE_INTERVAL): Likewise.
>         * toplev.c (process_options): Issue warnings/errors for cases
>         not handled with -fstack-clash-protection.
>
>
>
>         * doc/invoke.texi (-fstack-clash-protection): Document new option.
>         (-fstack-check): Note additional problem with -fstack-check=generic.
>         Note that -fstack-check is primarily for Ada and refer users
>         to -fstack-clash-protection for stack-clash-protection.
>         Document new params for stack clash protection.
>
>
>
>
>         * toplev.c (process_options): Handle -fstack-clash-protection.
>
> testsuite/
>
>         * gcc.dg/stack-check-2.c: New test.
>         * lib/target-supports.exp
>         (check_effective_target_supports_stack_clash_protection): New function.
>         (check_effective_target_frame_pointer_for_non_leaf): Likewise.
>         (check_effective_target_caller_implicit_probes): Likewise.
>
>
>
> diff --git a/gcc/common.opt b/gcc/common.opt
> index e81165c..cfaf2bc 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -2306,6 +2306,11 @@ fstack-check
>  Common Alias(fstack-check=, specific, no)
>  Insert stack checking code into the program.  Same as -fstack-check=specific.
>
> +fstack-clash-protection
> +Common Report Var(flag_stack_clash_protection)
> +Insert code to probe each page of stack space as it is allocated to protect
> +from stack-clash style attacks
> +
>  fstack-limit
>  Common Var(common_deferred_options) Defer
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 3e5cee8..8095dc2 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -10128,6 +10128,20 @@ compilation without.  The value for compilation with profile feedback
>  needs to be more conservative (higher) in order to make tracer
>  effective.
>
> +@item stack-clash-protection-guard-size
> +The size (in bytes) of the stack guard.  Acceptable values are powers of 2
> +between 4096 and 134217728 and defaults to 4096.  Higher values may reduce the
> +number of explicit probes, but a value larger than the operating system
> +provided guard will leave code vulnerable to stack clash style attacks.
> +
> +@item stack-clash-protection-probe-interval
> +Stack clash protection involves probing stack space as it is allocated.  This
> +param controls the maximum distance (in bytes) between probes into the stack.
> +Acceptable values are powers of 2 between 4096 and 65536 and defaults to
> +4096.  Higher values may reduce the number of explicit probes, but a value
> +larger than the operating system provided guard will leave code vulnerable to
> +stack clash style attacks.
> +
>  @item max-cse-path-length
>
>  The maximum number of basic blocks on path that CSE considers.
> @@ -11333,7 +11347,8 @@ target support in the compiler but comes with the following drawbacks:
>  @enumerate
>  @item
>  Modified allocation strategy for large objects: they are always
> -allocated dynamically if their size exceeds a fixed threshold.
> +allocated dynamically if their size exceeds a fixed threshold.  Note this
> +may change the semantics of some code.
>
>  @item
>  Fixed limit on the size of the static frame of functions: when it is
> @@ -11348,6 +11363,25 @@ generic implementation, code performance is hampered.
>  Note that old-style stack checking is also the fallback method for
>  @samp{specific} if no target support has been added in the compiler.
>
> +@samp{-fstack-check=} is designed for Ada's needs to detect infinite recursion
> +and stack overflows.  @samp{specific} is an excellent choice when compiling
> +Ada code.  It is not generally sufficient to protect against stack-clash
> +attacks.  To protect against those you want @samp{-fstack-clash-protection}.
> +
> +@item -fstack-clash-protection
> +@opindex fstack-clash-protection
> +Generate code to prevent stack clash style attacks.  When this option is
> +enabled, the compiler will only allocate one page of stack space at a time
> +and each page is accessed immediately after allocation.  Thus, it prevents
> +allocations from jumping over any stack guard page provided by the
> +operating system.
> +
> +Most targets do not fully support stack clash protection.  However, on
> +those targets @option{-fstack-clash-protection} will protect dynamic stack
> +allocations.  @option{-fstack-clash-protection} may also provide limited
> +protection for static stack allocations if the target supports
> +@option{-fstack-check=specific}.
> +
>  @item -fstack-limit-register=@var{reg}
>  @itemx -fstack-limit-symbol=@var{sym}
>  @itemx -fno-stack-limit
> diff --git a/gcc/flag-types.h b/gcc/flag-types.h
> index 5faade5..8874cba 100644
> --- a/gcc/flag-types.h
> +++ b/gcc/flag-types.h
> @@ -166,7 +166,14 @@ enum permitted_flt_eval_methods
>    PERMITTED_FLT_EVAL_METHODS_C11
>  };
>
> -/* Type of stack check.  */
> +/* Type of stack check.
> +
> +   Stack checking is designed to detect infinite recursion for Ada
> +   programs.  Furthermore stack checking tries to ensure that scenario
> +   that enough stack space is left to run a signal handler.
> +
> +   -fstack-check= does not prevent stack-clash style attacks.  For that
> +   you want -fstack-clash-protection.  */
>  enum stack_check_type
>  {
>    /* Do not check the stack.  */
> diff --git a/gcc/params.c b/gcc/params.c
> index fab0ffa..8afe4c4 100644
> --- a/gcc/params.c
> +++ b/gcc/params.c
> @@ -209,6 +209,11 @@ set_param_value (const char *name, int value,
>      error ("maximum value of parameter %qs is %u",
>            compiler_params[i].option,
>            compiler_params[i].max_value);
> +  else if ((strcmp (name, "stack-clash-protection-guard-size") == 0
> +           || strcmp (name, "stack-clash-protection-probe-interval") == 0)
> +          && exact_log2 (value) == -1)
> +    error ("value of parameter %qs must be a power of 2",
> +          compiler_params[i].option);
>    else
>      set_param_value_internal ((compiler_param) i, value,
>                               params, params_set, true);
> diff --git a/gcc/params.def b/gcc/params.def
> index 6b07518..2270031 100644
> --- a/gcc/params.def
> +++ b/gcc/params.def
> @@ -213,6 +213,16 @@ DEFPARAM(PARAM_STACK_FRAME_GROWTH,
>          "Maximal stack frame growth due to inlining (in percent).",
>          1000, 0, 0)
>
> +DEFPARAM(PARAM_STACK_CLASH_PROTECTION_GUARD_SIZE,
> +        "stack-clash-protection-guard-size",
> +        "Minimum size (in bytes) of the stack guard, must be a power of 2 >= 4096.",
> +        4096, 4096, 134217728)
> +
> +DEFPARAM(PARAM_STACK_CLASH_PROTECTION_PROBE_INTERVAL,
> +        "stack-clash-protection-probe-interval",
> +        "Interval (in bytes) in which to probe the stack.",
> +        4096, 1024, 65536)
> +
>  /* The GCSE optimization will be disabled if it would require
>     significantly more memory than this value.  */
>  DEFPARAM(PARAM_MAX_GCSE_MEMORY,
> diff --git a/gcc/testsuite/gcc.dg/stack-check-2.c b/gcc/testsuite/gcc.dg/stack-check-2.c
> new file mode 100644
> index 0000000..196c4bb
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/stack-check-2.c
> @@ -0,0 +1,66 @@
> +/* The goal here is to ensure that we never consider a call to a noreturn
> +   function as a potential tail call.
> +
> +   Right now GCC discovers potential tail calls by looking at the
> +   predecessors of the exit block.  A call to a non-return function
> +   has no successors and thus can never match that first filter.
> +
> +   But that could change one day and we want to catch it.  The problem
> +   is the compiler could potentially optimize a tail call to a nonreturn
> +   function, even if the caller has a frame.  That breaks the assumption
> +   that calls probe *sp when saving the return address that some targets
> +   depend on to elide stack probes.  */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fstack-clash-protection -fdump-tree-tailc -fdump-tree-optimized" } */
> +/* { dg-require-effective-target supports_stack_clash_protection } */
> +
> +extern void foo (void) __attribute__ ((__noreturn__));
> +
> +
> +void
> +test_direct_1 (void)
> +{
> +  foo ();
> +}
> +
> +void
> +test_direct_2 (void)
> +{
> +  return foo ();
> +}
> +
> +void (*indirect)(void)__attribute__ ((noreturn));
> +
> +
> +void
> +test_indirect_1 ()
> +{
> +  (*indirect)();
> +}
> +
> +void
> +test_indirect_2 (void)
> +{
> +  return (*indirect)();;
> +}
> +
> +
> +typedef void (*pvfn)() __attribute__ ((noreturn));
> +
> +void (*indirect_casted)(void);
> +
> +void
> +test_indirect_casted_1 ()
> +{
> +  (*(pvfn)indirect_casted)();
> +}
> +
> +void
> +test_indirect_casted_2 (void)
> +{
> +  return (*(pvfn)indirect_casted)();
> +}
> +/* { dg-final { scan-tree-dump-not "tail call" "tailc" } } */
> +/* { dg-final { scan-tree-dump-not "tail call" "optimized" } } */
> +
> diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
> index fe5e777..25c3c80 100644
> --- a/gcc/testsuite/lib/target-supports.exp
> +++ b/gcc/testsuite/lib/target-supports.exp
> @@ -8468,3 +8468,78 @@ proc check_effective_target_arm_coproc4_ok { } {
>      return [check_cached_effective_target arm_coproc4_ok \
>                 check_effective_target_arm_coproc4_ok_nocache]
>  }
> +
> +# Return 1 if the target has support for stack probing designed
> +# to avoid stack-clash style attacks.
> +#
> +# This is used to restrict the stack-clash mitigation tests to
> +# just those targets that have been explicitly supported.
> +#
> +# In addition to the prologue work on those targets, each target's
> +# properties should be described in the functions below so that
> +# tests do not become a mess of unreadable target conditions.
> +#
> +proc check_effective_target_supports_stack_clash_protection { } {
> +  if { [istarget aarch*-*-*] || [istarget x86_64-*-*]
> +       || [istarget i?86-*-*] || [istarget s390*-*-*]
> +       || [istarget powerpc*-*-*] || [istarget rs6000*-*-*] } {
> +       return 1
> +  }
> +  return 0
> +}
> +
> +# Return 1 if the target creates a frame pointer for non-leaf functions
> +# Note we ignore cases where we apply tail call optimization here.
> +proc check_effective_target_frame_pointer_for_non_leaf { } {
> +  if { [istarget aarch*-*-*] } {
> +       return 1
> +  }
> +  return 0
> +}
> +
> +# Return 1 if the target's calling sequence or its ABI
> +# create implicit stack probes at or prior to function entry.
> +proc check_effective_target_caller_implicit_probes { } {
> +
> +  # On x86/x86_64 the call instruction itself pushes the return
> +  # address onto the stack.  That is an implicit probe of *sp.
> +  if { [istarget x86_64-*-*] || [istarget i?86-*-*] } {
> +       return 1
> +  }
> +
> +  # On PPC, the ABI mandates that the address of the outer
> +  # frame be stored at *sp.  Thus each allocation of stack
> +  # space is itself an implicit probe of *sp.
> +  if { [istarget powerpc*-*-*] || [istarget rs6000*-*-*] } {
> +       return 1
> +  }
> +
> +  # s390's ABI has a register save area allocated by the
> +  # caller for use by the callee.  The mere existence does
> +  # not constitute a probe by the caller, but when the slots
> +  # used by the callee those stores are implicit probes.
> +  if { [istarget s390*-*-*] } {
> +       return 1
> +  }
> +
> +  # Not strictly true on aarch64, but we have agreed that we will
> +  # consider any function that pushes SP more than 3kbytes into
> +  # the guard page as broken.  This essentially means that we can
> +  # consider the aarch64 as having a caller implicit probe at
> +  # *(sp + 1k).
> +  if { [istarget aarch64*-*-*] } {
> +       return 1;
> +  }
> +
> +  return 0
> +}
> +
> +# Targets that potentially realign the stack pointer often cause residual
> +# stack allocations and make it difficult to elimination loops or residual
> +# allocations for dynamic stack allocations
> +proc check_effective_target_callee_realigns_stack { } {
> +  if { [istarget x86_64-*-*] || [istarget i?86-*-*] } {
> +       return 1
> +  }
> +  return 0
> +}
> diff --git a/gcc/toplev.c b/gcc/toplev.c
> index e6c69a4..24a00df 100644
> --- a/gcc/toplev.c
> +++ b/gcc/toplev.c
> @@ -1591,6 +1591,26 @@ process_options (void)
>        flag_associative_math = 0;
>      }
>
> +  /* -fstack-clash-protection is not currently supported on targets
> +     where the stack grows up.  */
> +  if (flag_stack_clash_protection && !STACK_GROWS_DOWNWARD)
> +    {
> +      warning_at (UNKNOWN_LOCATION, 0,
> +                 "-fstack-clash_protection is not supproted on targets "
> +                 "where the stack grows from lower to higher addresses");
> +      flag_stack_clash_protection = 0;
> +    }
> +
> +  /* We can not support -fstack-check= and -fstack-clash-protection at
> +     the same time.  */
> +  if (flag_stack_check != NO_STACK_CHECK && flag_stack_clash_protection)
> +    {
> +      warning_at (UNKNOWN_LOCATION, 0,
> +                 "-fstack-check= and -fstack-clash_protection are mutually "
> +                 "exclusive.  Disabling -fstack-check=");
> +      flag_stack_check = NO_STACK_CHECK;
> +    }
> +
>    /* With -fcx-limited-range, we do cheap and quick complex arithmetic.  */
>    if (flag_cx_limited_range)
>      flag_complex_method = 0;
>

On 08/18/2017 08:01 AM, Richard Biener wrote:
> On Mon, Jul 31, 2017 at 7:35 AM, Jeff Law <law@redhat.com> wrote:
>> This patch introduces the stack clash protection options
>>
>> Changes since V2:
>>
>> Adds two new params.  The first controls the size of the guard area.
>> This controls the threshold for when a function prologue requires
>> probes.  The second controls the probing interval -- ie, once probes are
>> needed, how often do we emit them.  These are really meant more for
>> developers to experiment with than users.  Regardless I did go ahead and
>> document them./PARAM
>>
>> It also adds some sanity checking WRT combining stack clash protection
>> with -fstack-check.
> 
> diff --git a/gcc/params.c b/gcc/params.c
> index fab0ffa..8afe4c4 100644
> --- a/gcc/params.c
> +++ b/gcc/params.c
> @@ -209,6 +209,11 @@ set_param_value (const char *name, int value,
>      error ("maximum value of parameter %qs is %u",
>     compiler_params[i].option,
>     compiler_params[i].max_value);
> +  else if ((strcmp (name, "stack-clash-protection-guard-size") == 0
> +    || strcmp (name, "stack-clash-protection-probe-interval") == 0)
> +   && exact_log2 (value) == -1)
> +    error ("value of parameter %qs must be a power of 2",
> +   compiler_params[i].option);
>    else
>      set_param_value_internal ((compiler_param) i, value,
>        params, params_set, true);
> 
> I don't like this.  Either use them as if they were power-of-two
> (floor_log2/ceil_log2 as appropriate) or simply make them take
> the logarithm instead (like -mincoming-stack-boundary and friends).
Yes.  I was torn on this for a variety of reasons, including the fact
that I don't actually expect anyone to be mucking with those :-)

Given we've already other stuff in log2 form, I'll use that -- it seems
less surprising to me than using floor/ceil.

> 
> Both -fstack-clash-protection and -fstack-check cannot be turned
> off per function.  This means they would need merging in lto-wrapper.
> The alternative is to mark them with 'Optimization' and allow per-function
> specification (like we do for -fstack-protector).
Do you have a strong preference here?  I'd tend to go with tweaking
lto-wrapper as we really don't want to have this stuff varying per-function.

Presumably in lto-wrapper we just have to detect that both were enabled
and do something sensible.  We drop -fstack-check in toplev.c when both
are specified, we could just as easily call that situation a fatal error
in both toplev.c and lto-wrapper.c

jeff

On Tue, Aug 22, 2017 at 5:52 PM, Jeff Law <law@redhat.com> wrote:
> On 08/18/2017 08:01 AM, Richard Biener wrote:
>> On Mon, Jul 31, 2017 at 7:35 AM, Jeff Law <law@redhat.com> wrote:
>>> This patch introduces the stack clash protection options
>>>
>>> Changes since V2:
>>>
>>> Adds two new params.  The first controls the size of the guard area.
>>> This controls the threshold for when a function prologue requires
>>> probes.  The second controls the probing interval -- ie, once probes are
>>> needed, how often do we emit them.  These are really meant more for
>>> developers to experiment with than users.  Regardless I did go ahead and
>>> document them./PARAM
>>>
>>> It also adds some sanity checking WRT combining stack clash protection
>>> with -fstack-check.
>>
>> diff --git a/gcc/params.c b/gcc/params.c
>> index fab0ffa..8afe4c4 100644
>> --- a/gcc/params.c
>> +++ b/gcc/params.c
>> @@ -209,6 +209,11 @@ set_param_value (const char *name, int value,
>>      error ("maximum value of parameter %qs is %u",
>>     compiler_params[i].option,
>>     compiler_params[i].max_value);
>> +  else if ((strcmp (name, "stack-clash-protection-guard-size") == 0
>> +    || strcmp (name, "stack-clash-protection-probe-interval") == 0)
>> +   && exact_log2 (value) == -1)
>> +    error ("value of parameter %qs must be a power of 2",
>> +   compiler_params[i].option);
>>    else
>>      set_param_value_internal ((compiler_param) i, value,
>>        params, params_set, true);
>>
>> I don't like this.  Either use them as if they were power-of-two
>> (floor_log2/ceil_log2 as appropriate) or simply make them take
>> the logarithm instead (like -mincoming-stack-boundary and friends).
> Yes.  I was torn on this for a variety of reasons, including the fact
> that I don't actually expect anyone to be mucking with those :-)
>
> Given we've already other stuff in log2 form, I'll use that -- it seems
> less surprising to me than using floor/ceil.
>
>>
>> Both -fstack-clash-protection and -fstack-check cannot be turned
>> off per function.  This means they would need merging in lto-wrapper.
>> The alternative is to mark them with 'Optimization' and allow per-function
>> specification (like we do for -fstack-protector).
> Do you have a strong preference here?  I'd tend to go with tweaking
> lto-wrapper as we really don't want to have this stuff varying per-function.

Any reason?  I see no technical one at least, it might break IPA assumptions
you make when instrumenting?  I guess you just don't want to think about
the implications when mixing them (or mixing -fstack-clash-protection
with -fno-stack-clash-protection)?

> Presumably in lto-wrapper we just have to detect that both were enabled
> and do something sensible.  We drop -fstack-check in toplev.c when both
> are specified, we could just as easily call that situation a fatal error
> in both toplev.c and lto-wrapper.c

So what happens if you LTO link Ada (-fstack-check) with C
(-fstack-clash-protection)?

The user is already allowed to (without LTO) specify things on a CU granularity.

Richard.
>
> jeff
>
>

On 08/23/2017 03:07 AM, Richard Biener wrote:

>>> Both -fstack-clash-protection and -fstack-check cannot be turned
>>> off per function.  This means they would need merging in lto-wrapper.
>>> The alternative is to mark them with 'Optimization' and allow per-function
>>> specification (like we do for -fstack-protector).
>> Do you have a strong preference here?  I'd tend to go with tweaking
>> lto-wrapper as we really don't want to have this stuff varying per-function.
> 
> Any reason?  I see no technical one at least, it might break IPA assumptions
> you make when instrumenting?  I guess you just don't want to think about
> the implications when mixing them (or mixing -fstack-clash-protection
> with -fno-stack-clash-protection)?
On some targets (s390 for example), a stack guard jump can be
accomplished by a combination of caller and callee stack adjustments --
even though neither the caller nor the callee have a large enough
adjustment individually to jump the guard.

So a mis-informed developer could audit their code, declare various
functions as safe & turn off protections on a per-function basis based
on that analysis.  But that would be mis-guided as it opens them up to a
potential attack where the combination of caller and callee stack
adjustments are used to jump the guard.





> 
>> Presumably in lto-wrapper we just have to detect that both were enabled
>> and do something sensible.  We drop -fstack-check in toplev.c when both
>> are specified, we could just as easily call that situation a fatal error
>> in both toplev.c and lto-wrapper.c
> 
> So what happens if you LTO link Ada (-fstack-check) with C
> (-fstack-clash-protection)?
When in the Ada code you'll be mostly protected from stack clashes and
you can detect stack overflows and catch the signal properly.  When in C
code, you'd be fully protected from stack clash, but not necessarily
able to take a signal.

At the boundaries where you call from Ada to C, you're potentially
vulnerable to stack clash on architectures like aarch64/s390.  When
calling from C into Ada, you're not guaranteed to be able to handle a
signal when you overflow the stack.

> The user is already allowed to (without LTO) specify things on a CU
granularity.
True.  And you end up with a mixture of protections depending on what
code is executing or what boundary you're crossing.


Jeff

On August 23, 2017 6:36:32 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>On 08/23/2017 03:07 AM, Richard Biener wrote:
>
>>>> Both -fstack-clash-protection and -fstack-check cannot be turned
>>>> off per function.  This means they would need merging in
>lto-wrapper.
>>>> The alternative is to mark them with 'Optimization' and allow
>per-function
>>>> specification (like we do for -fstack-protector).
>>> Do you have a strong preference here?  I'd tend to go with tweaking
>>> lto-wrapper as we really don't want to have this stuff varying
>per-function.
>> 
>> Any reason?  I see no technical one at least, it might break IPA
>assumptions
>> you make when instrumenting?  I guess you just don't want to think
>about
>> the implications when mixing them (or mixing -fstack-clash-protection
>> with -fno-stack-clash-protection)?
>On some targets (s390 for example), a stack guard jump can be
>accomplished by a combination of caller and callee stack adjustments --
>even though neither the caller nor the callee have a large enough
>adjustment individually to jump the guard.
>
>So a mis-informed developer could audit their code, declare various
>functions as safe & turn off protections on a per-function basis based
>on that analysis.  But that would be mis-guided as it opens them up to
>a
>potential attack where the combination of caller and callee stack
>adjustments are used to jump the guard.
>
>
>
>
>
>> 
>>> Presumably in lto-wrapper we just have to detect that both were
>enabled
>>> and do something sensible.  We drop -fstack-check in toplev.c when
>both
>>> are specified, we could just as easily call that situation a fatal
>error
>>> in both toplev.c and lto-wrapper.c
>> 
>> So what happens if you LTO link Ada (-fstack-check) with C
>> (-fstack-clash-protection)?
>When in the Ada code you'll be mostly protected from stack clashes and
>you can detect stack overflows and catch the signal properly.  When in
>C
>code, you'd be fully protected from stack clash, but not necessarily
>able to take a signal.
>
>At the boundaries where you call from Ada to C, you're potentially
>vulnerable to stack clash on architectures like aarch64/s390.  When
>calling from C into Ada, you're not guaranteed to be able to handle a
>signal when you overflow the stack.
>
>> The user is already allowed to (without LTO) specify things on a CU
>granularity.
>True.  And you end up with a mixture of protections depending on what
>code is executing or what boundary you're crossing.

So the natural thing for LTO ist to make it function granular. That way you don't need any magic in LTO-wrapper. 

Richard. 

>
>Jeff

On 08/23/2017 10:39 AM, Richard Biener wrote:
> On August 23, 2017 6:36:32 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>> On 08/23/2017 03:07 AM, Richard Biener wrote:
>>
>>>>> Both -fstack-clash-protection and -fstack-check cannot be turned
>>>>> off per function.  This means they would need merging in
>> lto-wrapper.
>>>>> The alternative is to mark them with 'Optimization' and allow
>> per-function
>>>>> specification (like we do for -fstack-protector).
>>>> Do you have a strong preference here?  I'd tend to go with tweaking
>>>> lto-wrapper as we really don't want to have this stuff varying
>> per-function.
>>>
>>> Any reason?  I see no technical one at least, it might break IPA
>> assumptions
>>> you make when instrumenting?  I guess you just don't want to think
>> about
>>> the implications when mixing them (or mixing -fstack-clash-protection
>>> with -fno-stack-clash-protection)?
>> On some targets (s390 for example), a stack guard jump can be
>> accomplished by a combination of caller and callee stack adjustments --
>> even though neither the caller nor the callee have a large enough
>> adjustment individually to jump the guard.
>>
>> So a mis-informed developer could audit their code, declare various
>> functions as safe & turn off protections on a per-function basis based
>> on that analysis.  But that would be mis-guided as it opens them up to
>> a
>> potential attack where the combination of caller and callee stack
>> adjustments are used to jump the guard.
>>
>>
>>
>>
>>
>>>
>>>> Presumably in lto-wrapper we just have to detect that both were
>> enabled
>>>> and do something sensible.  We drop -fstack-check in toplev.c when
>> both
>>>> are specified, we could just as easily call that situation a fatal
>> error
>>>> in both toplev.c and lto-wrapper.c
>>>
>>> So what happens if you LTO link Ada (-fstack-check) with C
>>> (-fstack-clash-protection)?
>> When in the Ada code you'll be mostly protected from stack clashes and
>> you can detect stack overflows and catch the signal properly.  When in
>> C
>> code, you'd be fully protected from stack clash, but not necessarily
>> able to take a signal.
>>
>> At the boundaries where you call from Ada to C, you're potentially
>> vulnerable to stack clash on architectures like aarch64/s390.  When
>> calling from C into Ada, you're not guaranteed to be able to handle a
>> signal when you overflow the stack.
>>
>>> The user is already allowed to (without LTO) specify things on a CU
>> granularity.
>> True.  And you end up with a mixture of protections depending on what
>> code is executing or what boundary you're crossing.
> 
> So the natural thing for LTO ist to make it function granular. That way you don't need any magic in LTO-wrapper. 
Done.
jeff