Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

[Kostya Serebryany <kcc@google.com>]

On Wed, Feb 20, 2019 at 1:40 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Wed, Feb 20, 2019 at 10:13 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > In the example in https://bugs.llvm.org/show_bug.cgi?id=38809#c12
> > (https://godbolt.org/z/ylsGSQ) there is no inlining, yet clang uses
> > over ten times as much stack space as gcc, for reasons I still
> > can't explain. My assumption right now is that the underlying bug
> > causes most of the problems with excessive stack usage in
> > allmodconfig kernels.
>
> Here is an even more minimal example:
>
> struct s { int i[5]; } f(void);
> void g(void) { f(); f();}

On this example I can see some stupidity that clang/asan is doing.
Let me try fixing it and see if it helps bigger cases.
Thanks for reducing the case!

This is the input we get in the asan instrumentation:

; Function Attrs: noinline nounwind optnone sanitize_address uwtable
define dso_local void @g() #0 {
entry:
  %tmp = alloca %struct.s, align 4  <<<<<<<<<<<<<<<<<<<<<<<
  %tmp1 = alloca %struct.s, align 4
  %0 = bitcast %struct.s* %tmp to i8*
  call void @llvm.lifetime.start.p0i8(i64 20, i8* %0) #3
  call void @f(%struct.s* sret %tmp)
  %1 = bitcast %struct.s* %tmp to i8* <<<<<<<<<<<<<<<<<<<<<
  call void @llvm.lifetime.end.p0i8(i64 20, i8* %1) #3
  %2 = bitcast %struct.s* %tmp1 to i8*
  call void @llvm.lifetime.start.p0i8(i64 20, i8* %2) #3  <<<<<<<<<<<<<
  call void @f(%struct.s* sret %tmp1)
  %3 = bitcast %struct.s* %tmp1 to i8*
  call void @llvm.lifetime.end.p0i8(i64 20, i8* %3) #3
  ret void
}

the stack variables are not *really* used, but since they are "used"
inside the lifetime markers they are not eliminated by asan,
and so asan instruments them, after which no one can remove them any more...




>
> https://godbolt.org/z/d_KWkh
>
> It's clear that clang does /something/ here when asan-stack=1 is
> set, but I fail to see what it is, or why that is necessary.
>
> The output of clang with asan-stack=0 is the expected
> code, and basically identical to what gcc produces with or
> without asan-stack.
>
>       Arnd