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 <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Kees Cook <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Link: https://lore.kernel.org/r/[email protected]

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