[PATCH v3] Use __executable_start as the lowest address for profiling [BZ #28153]

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

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On Sat, Jul 31, 2021 at 10:11 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > On Sat, Jul 31, 2021 at 9:36 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * H. J. Lu via Libc-alpha:
> >>
> >> > diff --git a/sysdeps/aarch64/start.S b/sysdeps/aarch64/start.S
> >> > index 417da8802b..e46e01ed0b 100644
> >> > --- a/sysdeps/aarch64/start.S
> >> > +++ b/sysdeps/aarch64/start.S
> >> > @@ -42,7 +42,7 @@
> >> >                                       NULL
> >> >   */
> >> >
> >> > -     .text
> >> > +     .section .text.unlikely,"ax",%progbits
> >> >  ENTRY(_start)
> >> >       /* Create an initial frame with 0 LR and FP */
> >> >       cfi_undefined (x30)
> >>
> >> I don't think it's correct to place code that runs during every process
> >> start into .text.unlikely.  Surely we can avoid that page fault.
> >>
> >> Can we fix the ENTRY_POINT assumption in profiling instead?
> >
> > We can do
> >
> > diff --git a/csu/gmon-start.c b/csu/gmon-start.c
> > index b3432885b3..83322fd586 100644
> > --- a/csu/gmon-start.c
> > +++ b/csu/gmon-start.c
> > @@ -48,7 +48,7 @@
> >  #ifdef ENTRY_POINT_DECL
> >  ENTRY_POINT_DECL(extern)
> >  #else
> > -extern char ENTRY_POINT[];
> > +extern char entry_point[] asm (__SYMBOL_PREFIX "main");
> >  #endif
> >  extern char etext[];
> >
> > @@ -56,7 +56,7 @@ extern char etext[];
> >  # ifdef ENTRY_POINT_DECL
> >  #  define TEXT_START ENTRY_POINT
> >  # else
> > -#  define TEXT_START &ENTRY_POINT
> > +#  define TEXT_START &entry_point
> >  # endif
> >  #endif
> >
> > But this may only work with BFD linker which places .text.startup
> > section before .text section.
>
> Can we get the linker to emit a symbol at the start of the text section?

We already have __executable_start which is pretty close to what we
need.   Like this.

> Like it does for orphan sections?
>
> Then we can use a weak symbol reference in gmon-start.c and use the new
> symbol if it is available.
>
> Thanks,
> Florian
>


-- 
H.J.

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