stack: Replace "o" output with "r" input constraint

mainline inclusion
from mainline-v5.13-rc1
commit 2515dd6c
category: featrue
bugzilla: https://gitee.com/openeuler/kernel/issues/I5YQ6Z
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2515dd6ce8e545b0b2eece84920048ef9ed846c4

--------------------------------

"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: 39218ff4 ("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


Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: GONG Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>