| Message ID | 20200909171542.3673449-1-yhs@fb.com |
|---|---|
| State | Accepted |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | [bpf-next,v3] selftests/bpf: fix test_sysctl_loop{1,2} failure due to clang change | expand |
On Wed, Sep 9, 2020 at 10:16 AM Yonghong Song <yhs@fb.com> wrote: > > Andrii reported that with latest clang, when building selftests, we have > error likes: > error: progs/test_sysctl_loop1.c:23:16: in function sysctl_tcp_mem i32 (%struct.bpf_sysctl*): > Looks like the BPF stack limit of 512 bytes is exceeded. > Please move large on stack variables into BPF per-cpu array map. > > The error is triggered by the following LLVM patch: > https://reviews.llvm.org/D87134 > > For example, the following code is from test_sysctl_loop1.c: > static __always_inline int is_tcp_mem(struct bpf_sysctl *ctx) > { > volatile char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string"; > ... > } > Without the above LLVM patch, the compiler did optimization to load the string > (59 bytes long) with 7 64bit loads, 1 8bit load and 1 16bit load, > occupying 64 byte stack size. > > With the above LLVM patch, the compiler only uses 8bit loads, but subregister is 32bit. > So stack requirements become 4 * 59 = 236 bytes. Together with other stuff on > the stack, total stack size exceeds 512 bytes, hence compiler complains and quits. > > To fix the issue, removing "volatile" key word or changing "volatile" to > "const"/"static const" does not work, the string is put in .rodata.str1.1 section, > which libbpf did not process it and errors out with > libbpf: elf: skipping unrecognized data section(6) .rodata.str1.1 > libbpf: prog 'sysctl_tcp_mem': bad map relo against '.L__const.is_tcp_mem.tcp_mem_name' > in section '.rodata.str1.1' > > Defining the string const as global variable can fix the issue as it puts the string constant > in '.rodata' section which is recognized by libbpf. In the future, when libbpf can process > '.rodata.str*.*' properly, the global definition can be changed back to local definition. > > Defining tcp_mem_name as a global, however, triggered a verifier failure. > ./test_progs -n 7/21 > libbpf: load bpf program failed: Permission denied > libbpf: -- BEGIN DUMP LOG --- > libbpf: > invalid stack off=0 size=1 > verification time 6975 usec > stack depth 160+64 > processed 889 insns (limit 1000000) max_states_per_insn 4 total_states > 14 peak_states 14 mark_read 10 > > libbpf: -- END LOG -- > libbpf: failed to load program 'sysctl_tcp_mem' > libbpf: failed to load object 'test_sysctl_loop2.o' > test_bpf_verif_scale:FAIL:114 > #7/21 test_sysctl_loop2.o:FAIL > This actually exposed a bpf program bug. In test_sysctl_loop{1,2}, we have code > like > const char tcp_mem_name[] = "<...long string...>"; > ... > char name[64]; > ... > for (i = 0; i < sizeof(tcp_mem_name); ++i) > if (name[i] != tcp_mem_name[i]) > return 0; > In the above code, if sizeof(tcp_mem_name) > 64, name[i] access may be > out of bound. The sizeof(tcp_mem_name) is 59 for test_sysctl_loop1.c and > 79 for test_sysctl_loop2.c. > > Without promotion-to-global change, old compiler generates code where > the overflowed stack access is actually filled with valid value, so hiding > the bpf program bug. With promotion-to-global change, the code is different, > more specifically, the previous loading constants to stack is gone, and > "name" occupies stack[-64:0] and overflow access triggers a verifier error. > To fix the issue, adjust "name" buffer size properly. > > Reported-by: Andrii Nakryiko <andriin@fb.com> > Signed-off-by: Yonghong Song <yhs@fb.com> > --- LGTM. Acked-by: Andrii Nakryiko <andriin@fb.com> > tools/testing/selftests/bpf/progs/test_sysctl_loop1.c | 4 ++-- > tools/testing/selftests/bpf/progs/test_sysctl_loop2.c | 4 ++-- > 2 files changed, 4 insertions(+), 4 deletions(-) > > Changelog: > v2 -> v3: > . using sizeof(tcp_mem_name) instead of hardcoded value for > local buf "name". (Andrii) > v1 -> v2: > . The tcp_mem_name change actually triggers a verifier failure due to > a bpf program bug. Fixing the bpf program bug can make test pass > with both old and latest llvm. (Alexei) > [...]
On Wed, Sep 9, 2020 at 11:18 AM Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote: > > On Wed, Sep 9, 2020 at 10:16 AM Yonghong Song <yhs@fb.com> wrote: > > > > Andrii reported that with latest clang, when building selftests, we have > > error likes: > > error: progs/test_sysctl_loop1.c:23:16: in function sysctl_tcp_mem i32 (%struct.bpf_sysctl*): > > Looks like the BPF stack limit of 512 bytes is exceeded. > > Please move large on stack variables into BPF per-cpu array map. > > > > Reported-by: Andrii Nakryiko <andriin@fb.com> > > Signed-off-by: Yonghong Song <yhs@fb.com> > > --- > > LGTM. > > Acked-by: Andrii Nakryiko <andriin@fb.com> Applied. Thanks
diff --git a/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c b/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c index 458b0d69133e..553a282d816a 100644 --- a/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c +++ b/tools/testing/selftests/bpf/progs/test_sysctl_loop1.c @@ -18,11 +18,11 @@ #define MAX_ULONG_STR_LEN 7 #define MAX_VALUE_STR_LEN (TCP_MEM_LOOPS * MAX_ULONG_STR_LEN) +const char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string"; static __always_inline int is_tcp_mem(struct bpf_sysctl *ctx) { - volatile char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string"; unsigned char i; - char name[64]; + char name[sizeof(tcp_mem_name)]; int ret; memset(name, 0, sizeof(name)); diff --git a/tools/testing/selftests/bpf/progs/test_sysctl_loop2.c b/tools/testing/selftests/bpf/progs/test_sysctl_loop2.c index b2e6f9b0894d..2b64bc563a12 100644 --- a/tools/testing/selftests/bpf/progs/test_sysctl_loop2.c +++ b/tools/testing/selftests/bpf/progs/test_sysctl_loop2.c @@ -18,11 +18,11 @@ #define MAX_ULONG_STR_LEN 7 #define MAX_VALUE_STR_LEN (TCP_MEM_LOOPS * MAX_ULONG_STR_LEN) +const char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string_to_stress_byte_loop"; static __attribute__((noinline)) int is_tcp_mem(struct bpf_sysctl *ctx) { - volatile char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string_to_stress_byte_loop"; unsigned char i; - char name[64]; + char name[sizeof(tcp_mem_name)]; int ret; memset(name, 0, sizeof(name));
Andrii reported that with latest clang, when building selftests, we have error likes: error: progs/test_sysctl_loop1.c:23:16: in function sysctl_tcp_mem i32 (%struct.bpf_sysctl*): Looks like the BPF stack limit of 512 bytes is exceeded. Please move large on stack variables into BPF per-cpu array map. The error is triggered by the following LLVM patch: https://reviews.llvm.org/D87134 For example, the following code is from test_sysctl_loop1.c: static __always_inline int is_tcp_mem(struct bpf_sysctl *ctx) { volatile char tcp_mem_name[] = "net/ipv4/tcp_mem/very_very_very_very_long_pointless_string"; ... } Without the above LLVM patch, the compiler did optimization to load the string (59 bytes long) with 7 64bit loads, 1 8bit load and 1 16bit load, occupying 64 byte stack size. With the above LLVM patch, the compiler only uses 8bit loads, but subregister is 32bit. So stack requirements become 4 * 59 = 236 bytes. Together with other stuff on the stack, total stack size exceeds 512 bytes, hence compiler complains and quits. To fix the issue, removing "volatile" key word or changing "volatile" to "const"/"static const" does not work, the string is put in .rodata.str1.1 section, which libbpf did not process it and errors out with libbpf: elf: skipping unrecognized data section(6) .rodata.str1.1 libbpf: prog 'sysctl_tcp_mem': bad map relo against '.L__const.is_tcp_mem.tcp_mem_name' in section '.rodata.str1.1' Defining the string const as global variable can fix the issue as it puts the string constant in '.rodata' section which is recognized by libbpf. In the future, when libbpf can process '.rodata.str*.*' properly, the global definition can be changed back to local definition. Defining tcp_mem_name as a global, however, triggered a verifier failure. ./test_progs -n 7/21 libbpf: load bpf program failed: Permission denied libbpf: -- BEGIN DUMP LOG --- libbpf: invalid stack off=0 size=1 verification time 6975 usec stack depth 160+64 processed 889 insns (limit 1000000) max_states_per_insn 4 total_states 14 peak_states 14 mark_read 10 libbpf: -- END LOG -- libbpf: failed to load program 'sysctl_tcp_mem' libbpf: failed to load object 'test_sysctl_loop2.o' test_bpf_verif_scale:FAIL:114 #7/21 test_sysctl_loop2.o:FAIL This actually exposed a bpf program bug. In test_sysctl_loop{1,2}, we have code like const char tcp_mem_name[] = "<...long string...>"; ... char name[64]; ... for (i = 0; i < sizeof(tcp_mem_name); ++i) if (name[i] != tcp_mem_name[i]) return 0; In the above code, if sizeof(tcp_mem_name) > 64, name[i] access may be out of bound. The sizeof(tcp_mem_name) is 59 for test_sysctl_loop1.c and 79 for test_sysctl_loop2.c. Without promotion-to-global change, old compiler generates code where the overflowed stack access is actually filled with valid value, so hiding the bpf program bug. With promotion-to-global change, the code is different, more specifically, the previous loading constants to stack is gone, and "name" occupies stack[-64:0] and overflow access triggers a verifier error. To fix the issue, adjust "name" buffer size properly. Reported-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Yonghong Song <yhs@fb.com> --- tools/testing/selftests/bpf/progs/test_sysctl_loop1.c | 4 ++-- tools/testing/selftests/bpf/progs/test_sysctl_loop2.c | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-) Changelog: v2 -> v3: . using sizeof(tcp_mem_name) instead of hardcoded value for local buf "name". (Andrii) v1 -> v2: . The tcp_mem_name change actually triggers a verifier failure due to a bpf program bug. Fixing the bpf program bug can make test pass with both old and latest llvm. (Alexei)