[cfe-commits] [PATCH] Fix support for naked C/C++ functions writing to unexpected memory regions in debug builds

[Eli Friedman]

2012/11/6 Jeffrey Lim <jeff at lim.com.au>:
>
> First time post to this list -- I think I posted to the wrong place before
> (cfe-dev). Hopefully I'm right this time.
>
> The problem I was noticing is that a naked function would cause very
> unexpected behaviour. I simplified it to this test case:
>
> test.c:
>
> __attribute__((naked)) void NakedTest(int value, int value2)
> {
> asm("");
> }
>
>
> clang -S test.cpp
> test.s:
>
> .section __TEXT,__text,regular,pure_instructions
> .globl __Z9NakedTestii
> .align 4, 0x90
> __Z9NakedTestii:                        ## @_Z9NakedTestii
> .cfi_startproc
> ## BB#0:
> movl %edi, -4(%rbp)
> movl %esi, -8(%rbp)
> ## InlineAsm Start
> ## InlineAsm End
> ret
> .cfi_endproc
>
>
> clang -flto -S test.cpp
> test.s:
>
> define void @_Z9NakedTestii(i32 %value, i32 %value2) nounwind uwtable
> noinline ssp naked {
> entry:
>   %value.addr = alloca i32, align 4
>   %value2.addr = alloca i32, align 4
>   store i32 %value, i32* %value.addr, align 4
>   store i32 %value2, i32* %value2.addr, align 4
>   call void asm sideeffect "", "~{dirflag},~{fpsr},~{flags}"() nounwind,
> !srcloc !0
>   ret void
> }
>
> The bolded instructions above are generated in debug builds, causing
> problems. This happens in all architectures that I've tested on (ARM, x86,
> x64)
>
>
> I've attached a patch file which I fixes the problem. I don't know if I've
> done it the right way (I only downloaded and looked at the source for clang
> for the first time 2 hours ago), but here's the newer result:
>
> clang -S test.cpp
> test.s:
> .section __TEXT,__text,regular,pure_instructions
> .globl __Z9NakedTestii
> .align 4, 0x90
> __Z9NakedTestii:                        ## @_Z9NakedTestii
> .cfi_startproc
> ## BB#0:                                ## %entry
> ## InlineAsm Start
> ## InlineAsm End
> ret
> .cfi_endproc
>
>
> clang -flto -S test.cpp
> test.s:
>
> define void @_Z9NakedTestii(i32, i32) nounwind uwtable noinline ssp naked {
> entry:
>   call void asm sideeffect "", "~{dirflag},~{fpsr},~{flags}"() nounwind,
> !srcloc !0
>   ret void
> }
>
> Any comments, or alternative fixes to this problem would be greatly
> appreciated!

I would like to see a more principled approach here... scattering
checks for the naked attribute all over the function emission code
seems like a bad idea.  (clang with just your patch doesn't handle,
for example, the case where the return type is not void consistently,
the case where the function in question is a member function, etc.)

-Eli