[v2] arm64: signal: Report signal frame size to userspace via auxv

Message ID 1526047851-14807-1-git-send-email-Dave.Martin@arm.com
State New
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  • [v2] arm64: signal: Report signal frame size to userspace via auxv
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Commit Message

Dave Martin May 11, 2018, 2:10 p.m.
Stateful CPU architecture extensions may require the signal frame
to grow to a size that exceeds the arch's MINSIGSTKSZ #define.
However, changing this #define is an ABI break.

To allow userspace the option of determining the signal frame size
in a more forwards-compatible way, this patch adds a new auxv entry
tagged with AT_MINSIGSTKSZ, which provides the maximum signal frame
size that the process can observe during its lifetime.

If AT_MINSIGSTKSZ is absent from the aux vector, the caller can
assume that the MINSIGSTKSZ #define is sufficient.  This allows for
a consistent interface with older kernels that do not provide
AT_MINSIGSTKSZ.

The idea is that libc could expose this via sysconf() or some
similar mechanism.

There is deliberately no AT_SIGSTKSZ.  The kernel knows nothing
about userspace's own stack overheads and should not pretend to
know.

For arm64:

The primary motivation for this interface is the Scalable Vector
Extension, which can require at least 4KB or so of extra space
in the signal frame for the largest hardware implementations.

To determine the correct value, a "Christmas tree" mode (via the
add_all argument) is added to setup_sigframe_layout(), to simulate
addition of all possible records to the signal frame at maximum
possible size.

If this procedure goes wrong somehow, resulting in a stupidly large
frame layout and hence failure of sigframe_alloc() to allocate a
record to the frame, then this is indicative of a kernel bug: the
kernel's internal SIGFRAME_MAXSZ is supposed to sanity-check
against generting frames that we consider _impossibly_ large.  In
this case, SIGSTKSZ is returned as a "reasonable guess that is at
least bigger than MINSIGSTKSZ" and we WARN().

For arm64 SVE:

The SVE context block in the signal frame needs to be considered
too when computing the maximum possible signal frame size.

Because the size of this block depends on the vector length, this
patch computes the size based not on the thread's current vector
length but instead on the maximum possible vector length: this
determines the maximum size of SVE context block that can be
observed in any signal frame for the lifetime of the process.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Alex Bennée <alex.bennee@linaro.org>

---

Changes since v1 [1]:

 * Merged the SVE frame size computation into this patch.  It
   no longer makes sense for it to be separate, and without the
   latter patch an incorrect overall value is computed anyway.
   Best to avoid this.

Suggested by Mark Rutland:

 * Hook the minsigstksz initialisation into the end of cpufeatures
   setup so that we don't have to worry about parallelism.  It's
   reasonable to assume that the sigframe size is known by this point
   (and currently the case).


[1] [PATCH 0/2] arm64: Report signal frame size to userspace via auxv
http://lists.infradead.org/pipermail/linux-arm-kernel/2018-May/576269.html

---
 arch/arm64/include/asm/elf.h         | 11 ++++++++
 arch/arm64/include/asm/processor.h   |  5 ++++
 arch/arm64/include/uapi/asm/auxvec.h |  3 ++-
 arch/arm64/kernel/cpufeature.c       |  1 +
 arch/arm64/kernel/signal.c           | 51 +++++++++++++++++++++++++++++++-----
 5 files changed, 63 insertions(+), 8 deletions(-)

Patch

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index fac1c4d..dc32adb 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -24,6 +24,11 @@ 
 #include <asm/ptrace.h>
 #include <asm/user.h>
 
+#ifndef __ASSEMBLY__
+#include <linux/bug.h>
+#include <asm/processor.h> /* for signal_minsigstksz, used by ARCH_DLINFO */
+#endif
+
 /*
  * AArch64 static relocation types.
  */
@@ -146,8 +151,14 @@  typedef struct user_fpsimd_state elf_fpregset_t;
 /* update AT_VECTOR_SIZE_ARCH if the number of NEW_AUX_ENT entries changes */
 #define ARCH_DLINFO							\
 do {									\
+	int minsigstksz = signal_minsigstksz;				\
+									\
+	if (WARN_ON(minsigstksz <= 0))					\
+		minsigstksz = MINSIGSTKSZ;				\
+									\
 	NEW_AUX_ENT(AT_SYSINFO_EHDR,					\
 		    (elf_addr_t)current->mm->context.vdso);		\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, minsigstksz);			\
 } while (0)
 
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 7675989..6f60e92 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -35,6 +35,8 @@ 
 #ifdef __KERNEL__
 
 #include <linux/build_bug.h>
+#include <linux/cache.h>
+#include <linux/init.h>
 #include <linux/stddef.h>
 #include <linux/string.h>
 
@@ -244,6 +246,9 @@  void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused);
 void cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused);
 void cpu_clear_disr(const struct arm64_cpu_capabilities *__unused);
 
+extern int __ro_after_init signal_minsigstksz;	/* user signal frame size */
+extern void __init minsigstksz_setup(void);
+
 /* Userspace interface for PR_SVE_{SET,GET}_VL prctl()s: */
 #define SVE_SET_VL(arg)	sve_set_current_vl(arg)
 #define SVE_GET_VL()	sve_get_current_vl()
diff --git a/arch/arm64/include/uapi/asm/auxvec.h b/arch/arm64/include/uapi/asm/auxvec.h
index ec0a86d..a35797e 100644
--- a/arch/arm64/include/uapi/asm/auxvec.h
+++ b/arch/arm64/include/uapi/asm/auxvec.h
@@ -19,7 +19,8 @@ 
 
 /* vDSO location */
 #define AT_SYSINFO_EHDR	33
+#define AT_MINSIGSTKSZ	34	/* stack needed for signal delivery */
 
-#define AT_VECTOR_SIZE_ARCH 1 /* entries in ARCH_DLINFO */
+#define AT_VECTOR_SIZE_ARCH 2 /* entries in ARCH_DLINFO */
 
 #endif
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9d1b06d..0e0b53d 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1619,6 +1619,7 @@  void __init setup_cpu_features(void)
 		pr_info("emulated: Privileged Access Never (PAN) using TTBR0_EL1 switching\n");
 
 	sve_setup();
+	minsigstksz_setup();
 
 	/* Advertise that we have computed the system capabilities */
 	set_sys_caps_initialised();
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 154b7d3..ae8d4ea 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -17,6 +17,7 @@ 
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <linux/cache.h>
 #include <linux/compat.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
@@ -570,8 +571,15 @@  asmlinkage long sys_rt_sigreturn(struct pt_regs *regs)
 	return 0;
 }
 
-/* Determine the layout of optional records in the signal frame */
-static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
+/*
+ * Determine the layout of optional records in the signal frame
+ *
+ * add_all: if true, lays out the biggest possible signal frame for
+ *	this task; otherwise, generates a layout for the current state
+ *	of the task.
+ */
+static int setup_sigframe_layout(struct rt_sigframe_user_layout *user,
+				 bool add_all)
 {
 	int err;
 
@@ -581,7 +589,7 @@  static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 		return err;
 
 	/* fault information, if valid */
-	if (current->thread.fault_code) {
+	if (add_all || current->thread.fault_code) {
 		err = sigframe_alloc(user, &user->esr_offset,
 				     sizeof(struct esr_context));
 		if (err)
@@ -591,8 +599,18 @@  static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 	if (system_supports_sve()) {
 		unsigned int vq = 0;
 
-		if (test_thread_flag(TIF_SVE))
-			vq = sve_vq_from_vl(current->thread.sve_vl);
+		if (add_all || test_thread_flag(TIF_SVE)) {
+			int vl = sve_max_vl;
+
+			if (!add_all)
+				vl = current->thread.sve_vl;
+
+			/* Fail safe if something wasn't initialised */
+			if (WARN_ON(!sve_vl_valid(vl)))
+				vl = SVE_VL_MIN;
+
+			vq = sve_vq_from_vl(vl);
+		}
 
 		err = sigframe_alloc(user, &user->sve_offset,
 				     SVE_SIG_CONTEXT_SIZE(vq));
@@ -603,7 +621,6 @@  static int setup_sigframe_layout(struct rt_sigframe_user_layout *user)
 	return sigframe_alloc_end(user);
 }
 
-
 static int setup_sigframe(struct rt_sigframe_user_layout *user,
 			  struct pt_regs *regs, sigset_t *set)
 {
@@ -701,7 +718,7 @@  static int get_sigframe(struct rt_sigframe_user_layout *user,
 	int err;
 
 	init_user_layout(user);
-	err = setup_sigframe_layout(user);
+	err = setup_sigframe_layout(user, false);
 	if (err)
 		return err;
 
@@ -936,3 +953,23 @@  asmlinkage void do_notify_resume(struct pt_regs *regs,
 		thread_flags = READ_ONCE(current_thread_info()->flags);
 	} while (thread_flags & _TIF_WORK_MASK);
 }
+
+int __ro_after_init signal_minsigstksz;
+
+/*
+ * Determine the stack space required for guaranteed signal devliery.
+ * This function is used to populate AT_MINSIGSTKSZ at process startup.
+ */
+void __init minsigstksz_setup(void)
+{
+	struct rt_sigframe_user_layout user;
+
+	init_user_layout(&user);
+
+	if (WARN_ON(setup_sigframe_layout(&user, true)))
+		signal_minsigstksz = SIGSTKSZ;
+	else
+		signal_minsigstksz = sigframe_size(&user) +
+			round_up(sizeof(struct frame_record), 16) +
+			16; /* max alignment padding */
+}