3.16.62-rc1 review patch. If anyone has any objections, please let me know.
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From: Andy Lutomirski luto@kernel.org
commit 715bd9d12f84d8f5cc8ad21d888f9bc304a8eb0b upstream.
The syscall fallbacks in the vDSO have incorrect asm constraints. They are not marked as writing to their outputs -- instead, they are marked as clobbering "memory", which is useless. In particular, gcc is smart enough to know that the timespec parameter hasn't escaped, so a memory clobber doesn't clobber it. And passing a pointer as an asm *input* does not tell gcc that the pointed-to value is changed.
Add in the fact that the asm instructions weren't volatile, and gcc was free to omit them entirely unless their sole output (the return value) is used. Which it is (phew!), but that stops happening with some upcoming patches.
As a trivial example, the following code:
void test_fallback(struct timespec *ts) { vdso_fallback_gettime(CLOCK_MONOTONIC, ts); }
compiles to:
00000000000000c0 <test_fallback>: c0: c3 retq
To add insult to injury, the RCX and R11 clobbers on 64-bit builds were missing.
The "memory" clobber is also unnecessary -- no ordering with respect to other memory operations is needed, but that's going to be fixed in a separate not-for-stable patch.
Fixes: 2aae950b21e4 ("x86_64: Add vDSO for x86-64 with gettimeofday/clock_gettime/getcpu") Signed-off-by: Andy Lutomirski luto@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Link: https://lkml.kernel.org/r/2c0231690551989d2fafa60ed0e7b5cc8b403908.153842229... [bwh: Backported to 3.16: adjust filename] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/x86/vdso/vclock_gettime.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-)
--- a/arch/x86/vdso/vclock_gettime.c +++ b/arch/x86/vdso/vclock_gettime.c @@ -50,8 +50,9 @@ extern u8 pvclock_page notrace static long vdso_fallback_gettime(long clock, struct timespec *ts) { long ret; - asm("syscall" : "=a" (ret) : - "0" (__NR_clock_gettime), "D" (clock), "S" (ts) : "memory"); + asm ("syscall" : "=a" (ret), "=m" (*ts) : + "0" (__NR_clock_gettime), "D" (clock), "S" (ts) : + "memory", "rcx", "r11"); return ret; }
@@ -59,8 +60,9 @@ notrace static long vdso_fallback_gtod(s { long ret;
- asm("syscall" : "=a" (ret) : - "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory"); + asm ("syscall" : "=a" (ret), "=m" (*tv), "=m" (*tz) : + "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : + "memory", "rcx", "r11"); return ret; }
@@ -143,12 +145,12 @@ notrace static long vdso_fallback_gettim { long ret;
- asm( + asm ( "mov %%ebx, %%edx \n" "mov %2, %%ebx \n" "call __kernel_vsyscall \n" "mov %%edx, %%ebx \n" - : "=a" (ret) + : "=a" (ret), "=m" (*ts) : "0" (__NR_clock_gettime), "g" (clock), "c" (ts) : "memory", "edx"); return ret; @@ -158,12 +160,12 @@ notrace static long vdso_fallback_gtod(s { long ret;
- asm( + asm ( "mov %%ebx, %%edx \n" "mov %2, %%ebx \n" "call __kernel_vsyscall \n" "mov %%edx, %%ebx \n" - : "=a" (ret) + : "=a" (ret), "=m" (*tv), "=m" (*tz) : "0" (__NR_gettimeofday), "g" (tv), "c" (tz) : "memory", "edx"); return ret;