Re: [PATCH v3 13/13] aarch64: fix _mcount for pac-ret

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 05/26/2020 08:33, Adhemerval Zanella via Libc-alpha wrote:
> 
> 
> On 15/05/2020 11:40, Szabolcs Nagy wrote:
> > gcc -pg with -mbranch-protection=pac-ret passes signed return address
> > to _mcount, so _mcount now has to always strip pac from the frompc
> > since that's from user code that may be built with pac-ret.
> > 
> > This is a backward incompatible _mcount abi change introduced by
> > return address signing support in gcc-7.
> 
> Which change are you referring about specifically?

gcc-7 introduced -msigned-return-address (which was
later deprecated by -mbranch-protection=pac-ret) and
the code generation with -pg is not compatible with
existing _mcount runtime implementations (because
now _mcount needs unconditional xpac)

i'm still trying to see if this _mcount change is
needed or we can change gcc to do the right thing
(and not pass signed address to _mcount), but i
need internal consensus about such abi break first.

> 
> Besides it, LGTM.
> 
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> 
> > 
> > TODO: fix -pg on the gcc side?
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94791
> > ---
> >  sysdeps/aarch64/machine-gmon.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/sysdeps/aarch64/machine-gmon.h b/sysdeps/aarch64/machine-gmon.h
> > index 730a23b781..328cbdda16 100644
> > --- a/sysdeps/aarch64/machine-gmon.h
> > +++ b/sysdeps/aarch64/machine-gmon.h
> > @@ -30,5 +30,5 @@ static inline void mcount_internal (u_long frompc, u_long selfpc)
> >  #define MCOUNT                                                    \
> >  void __mcount (void *frompc)                                      \
> >  {                                                                 \
> > -  mcount_internal ((u_long) frompc, (u_long) RETURN_ADDRESS (0)); \
> > +  mcount_internal ((u_long) strip_pac (frompc), (u_long) RETURN_ADDRESS (0)); \
> >  }
> > 

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