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authorNick Desaulniers <ndesaulniers@google.com>2021-04-20 02:17:41 +0300
committerThomas Gleixner <tglx@linutronix.de>2021-05-11 10:56:11 +0300
commit2515dd6ce8e545b0b2eece84920048ef9ed846c4 (patch)
tree3d6391ab0cdb68924c871a8e37b467683953f694
parent6efb943b8616ec53a5e444193dccf1af9ad627b5 (diff)
downloadlinux-2515dd6ce8e545b0b2eece84920048ef9ed846c4.tar.xz
stack: Replace "o" output with "r" input constraint
"o" isn't a common asm() constraint to use; it triggers an assertion in assert-enabled builds of LLVM that it's not recognized when targeting aarch64 (though it appears to fall back to "m"). It's fixed in LLVM 13 now, but there isn't really a good reason to use "o" in particular here. To avoid causing build issues for those using assert-enabled builds of earlier LLVM versions, the constraint needs changing. Instead, if the point is to retain the __builtin_alloca(), make ptr appear to "escape" via being an input to an empty inline asm block. This is preferable anyways, since otherwise this looks like a dead store. While the use of "r" was considered in https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/ it was only tested as an output (which looks like a dead store, and wasn't sufficient). Use "r" as an input constraint instead, which behaves correctly across compilers and architectures. Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Kees Cook <keescook@chromium.org> Tested-by: Nathan Chancellor <nathan@kernel.org> Reviewed-by: Nathan Chancellor <nathan@kernel.org> Link: https://reviews.llvm.org/D100412 Link: https://bugs.llvm.org/show_bug.cgi?id=49956 Link: https://lore.kernel.org/r/20210419231741.4084415-1-keescook@chromium.org
-rw-r--r--include/linux/randomize_kstack.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab663a9..bebc911161b6 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
u32 offset = raw_cpu_read(kstack_offset); \
u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \
/* Keep allocation even after "ptr" loses scope. */ \
- asm volatile("" : "=o"(*ptr) :: "memory"); \
+ asm volatile("" :: "r"(ptr) : "memory"); \
} \
} while (0)