From: Andy Lutomirski <luto@kernel.org> To: x86@kernel.org Cc: LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>, stable@vger.kernel.org Subject: [PATCH 1/2] x86/vdso: Fix asm constraints on vDSO syscall fallbacks Date: Mon, 1 Oct 2018 12:52:15 -0700 Message-ID: <2c0231690551989d2fafa60ed0e7b5cc8b403908.1538422295.git.luto@kernel.org> (raw) In-Reply-To: <cover.1538422295.git.luto@kernel.org> In-Reply-To: <cover.1538422295.git.luto@kernel.org> 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 I'll fix that in a separate not-for-stable patch. Cc: stable@vger.kernel.org Fixes: 2aae950b21e4 ("x86_64: Add vDSO for x86-64 with gettimeofday/clock_gettime/getcpu") Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/x86/entry/vdso/vclock_gettime.c | 18 ++++++++++-------- 1 file 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; -- 2.17.1
next prev parent reply index Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-01 19:52 [PATCH 0/2] x86/vdso: A bugfix and a selftest Andy Lutomirski 2018-10-01 19:52 ` Andy Lutomirski [this message] 2018-10-02 6:30 ` [tip:x86/urgent] x86/vdso: Fix asm constraints on vDSO syscall fallbacks tip-bot for Andy Lutomirski 2018-10-03 23:15 ` Andy Lutomirski 2018-10-01 19:52 ` [PATCH 2/2] selftests/x86: Add clock_gettime() tests to test_vdso Andy Lutomirski 2018-10-02 6:31 ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
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