Hello,
On Mon, 18 May 2020, Florian Weimer wrote:
> >> In glibc, we already have this:
> >>
> >> /* Used to disable stack protection in sensitive places, like ifunc
> >> resolvers and early static TLS init. */
> >> #ifdef HAVE_CC_NO_STACK_PROTECTOR
> >> # define inhibit_stack_protector \
> >> __attribute__ ((__optimize__ ("-fno-stack-protector")))
> >> #else
> >> # define inhibit_stack_protector
> >> #endif
> >>
> >> Is it broken?
> >
> > Depends on what your expectations are. It completely overrides all
> > options given on the command line (including things like
> > fno-omit-frame-pointer and -O2!). At least I was very surprised by that
> > even though the current docu can be read that way; if you're similarly
> > surprised, then yes, the above is broken, it does not only disable stack
> > protection (but also e.g. all optimizations, despite the attributes name
> > :-) ).
>
> Yes, that would qualify as broken.
>
> This is not what I observe with gcc-9.3.1-2.fc31.x86_64 from Fedora.
> -O2 still has an effect.
Indeed, I definitely remember an interaction with the attribute and -O{,2}
(or something that I interpreted as such) but it obviously isn't as simple
as plain disabling it, and right now I can't recreate the situation :-/
(It might be disabling of some further cmdline flags that I conflated in
my brain with "effect of -O2")
> So does -fcf-protection. -fno-omit-frame-pointer does not work for me at
> all for some reason, the frame pointer is always missing?
Not for me:
% cat simple.c
extern int bla(int *);
int
#ifdef ATTR
__attribute__((__optimize__ ("-fno-stack-protector")))
#endif
foo(int a, int b)
{
int c = b;
return a * 42 + bla(&c);
}
% gcc-9 -fno-omit-frame-pointer -O -S -o - tryme.c | grep bp
pushq %rbp
movq %rsp, %rbp
movl %esi, -20(%rbp)
leaq -20(%rbp), %rdi
popq %rbp
% gcc-9 -fstack-protector-all -fno-omit-frame-pointer -O -S -o - tryme.c | grep
bp
pushq %rbp
movq %rsp, %rbp
movq %rax, -24(%rbp)
movl %esi, -28(%rbp)
leaq -28(%rbp), %rdi
movq -24(%rbp), %rdx
popq %rbp
But using the attr:
% gcc-9 -DATTR -fstack-protector-all -fno-omit-frame-pointer -O -S -o - tryme.c
| grep bp
%
(gcc9 is gcc9-9.2.1+r275327-1.1.x86_64 on opensuse)
Ciao,
Michael.
