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authorAndy Lutomirski <luto@kernel.org>2018-10-01 22:52:15 +0300
committerThomas Gleixner <tglx@linutronix.de>2018-10-02 09:28:15 +0300
commit715bd9d12f84d8f5cc8ad21d888f9bc304a8eb0b (patch)
treea213618d05116da7b643f574d30072e46a7274c7 /arch/x86/entry
parent17b57b1883c1285f3d0dc2266e8f79286a7bef38 (diff)
downloadlinux-715bd9d12f84d8f5cc8ad21d888f9bc304a8eb0b.tar.xz
x86/vdso: Fix asm constraints on vDSO syscall fallbacks
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> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/2c0231690551989d2fafa60ed0e7b5cc8b403908.1538422295.git.luto@kernel.org
Diffstat (limited to 'arch/x86/entry')
-rw-r--r--arch/x86/entry/vdso/vclock_gettime.c18
1 files changed, 10 insertions, 8 deletions
diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d95c60..134e2d2e8add 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -43,8 +43,9 @@ extern u8 hvclock_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;
}
@@ -52,8 +53,9 @@ notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
{
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;
}
@@ -64,12 +66,12 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
{
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;
@@ -79,12 +81,12 @@ notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
{
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;