MaskRay added a comment.
In D148827#4424237 <https://reviews.llvm.org/D148827#4424237>, @uabelho wrote:
> Hi,
>
> A question: it seems like this messes with alignment on functions?
> So with input program al.c:
>
> __attribute__((aligned(64)))
> void alignedfn(void) {
> __asm("nop");
> }
> void alignedfn2(void) {
> __asm("nop");
> }
> int main(void){}
>
> if we compile with -fsanitize=function we get:
>
> -> clang al.c -o al.o -c -fsanitize=function
> -> nm al.o
> 0000000000000008 T alignedfn
> 0000000000000018 T alignedfn2
> 0000000000000028 T main
>
> So alignedfn and alignedfn2 doesn't seem to be aligned as we said.
> Without -fsanitize=function:
>
> -> clang al.c -o al.o -c
> -> nm al.o
> 0000000000000000 T alignedfn
> 0000000000000010 T alignedfn2
> 0000000000000020 T main
>
> I guess the data put before the functions get aligned but not the functions
> themselves. Should I write a ticket about this?
When `__attribute__((aligned(...)))` is used with an instrumentation that
places extra bytes before the function label, I think it is unclear what the
desired behavior should be.
This applies to many instructions including `-fsanitize=kcfi`,
`-fsanitize=function`, and `-fpatchable-function-entry=N,M` (M>0).
It's unclear whether this attribute applies to the first byte of the function
or the first intended instruction.
For example, with `-fpatchable-function-entry=2,2`, we can make
`st_value(foo)%64=2`.
gcc -c -fpatchable-function-entry=2,2 -xc =(printf
'__attribute__((aligned(64))) void foo() {}') -o a.o
clang -c -fpatchable-function-entry=2,2 -xc =(printf
'__attribute__((aligned(64))) void foo() {}') -o a.o
I acknowledge that
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html says "The
aligned attribute specifies a minimum alignment for the first instruction of
the function, measured in bytes.", but I think the GCC documentation doesn't
consider these instructions.
I consider Clang's behavior working as intended.
(Other angles: with PPC64 ELFv2, the local entry, the target destination of a
function call, is not aligned by `__attribute__((aligned(...)))`.
`__attribute__((aligned(...)))` is usually for performance reasons and has
different tradeoff with `-fsanitize=function`.)
Repository:
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https://reviews.llvm.org/D148827/new/
https://reviews.llvm.org/D148827
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