RE: [PATCH,resend] fix uninitialized variable in dynamic linker

[Matthew Fortune <Matthew dot Fortune at imgtec dot com>]

Ping^2

> 
> Matthew
> > -----Original Message-----
> > From: libc-alpha-owner@sourceware.org [mailto:libc-alpha-owner@sourceware.org] On Behalf
> > Of Matthew Fortune
> > Sent: 01 November 2016 13:01
> > To: Sandra Loosemore; Joseph Myers
> > Cc: libc-alpha@sourceware.org; Petar Jovanovic
> > Subject: [PATCH,resend] fix uninitialized variable in dynamic linker
> >
> > Sandra Loosemore <sandra@codesourcery.com> writes:
> > > On 10/26/2016 03:01 PM, Matthew Fortune wrote:
> > > > Joseph Myers <joseph@codesourcery.com> writes:
> > > >> On Wed, 26 Oct 2016, Matthew Fortune wrote:
> > > >>
> > > >>> +# ifdef ELF_MACHINE_INIT_MAP
> > > >>> +  ELF_MACHINE_INIT_MAP (bootstrap_map); # endif
> > > >>
> > > >> We don't encourage use of #ifdef like that.  It's better to have an
> > > >> inline function defined everywhere and used unconditionally, for
> > > >> which most systems have a dummy definition (see
> > > >> dl-machine-reject-phdr.h and elf_machine_reject_phdr_p for an example
> > > >> - if you have a header for a single function, you don't need to
> > > >> update lots of dl-machine.h headers, just add a generic version -
> > > >> which has the comments detailing the semantics of the function and
> > > when it's needed - and a MIPS version).
> > > >
> > > > Thanks Joseph. It's been a while since I did a glibc patch and
> > > > couldn't remember the recommended approach.
> > > >
> > > > Do you think I should add a whole new header for this? Or, since this
> > > > is directly related to the reject_phdr feature for MIPS and only MIPS
> > > > is affected then I could just add it to dl-machine-reject-phdr.h?
> > >
> > > Wouldn't it be easier and more maintainable just to unconditionally
> > > zero-initialize the structure, as I did in the original patch?
> >
> > (updated subject to follow Sandra's original thread)
> >
> > OK, given no further comments and your recommendation to stick with the
> > original. I'm reposting your original patch updated to remove minor
> > fuzz on applying.
> >
> > Roland originally suggested leaving the compiler to initialise the stack
> > allocated link_map structure but doing so would lead to an implicit
> > memset function call. Since calls are not possible until the end of the
> > _dl_start function then we can't do that.
> >
> > The data that gets zero'd increases from 1040 bytes to 1560 bytes for
> > MIPS n64 but the benefit of knowing the link_map is zero initialised
> > wherever it is created. I expect similar increases for the other
> > architectures that do not define PI_STATIC_AND_HIDDEN.
> >
> > I'm re-running tests but Sandra's original testing should still be valid.
> >
> > Thanks,
> > Matthew
> >
> > diff --git a/elf/rtld.c b/elf/rtld.c
> > index 647661c..a1b3a39 100644
> > --- a/elf/rtld.c
> > +++ b/elf/rtld.c
> > @@ -357,7 +357,7 @@ _dl_start (void *arg)
> >      HP_TIMING_NOW (info.start_time);
> >  #endif
> >
> > -  /* Partly clean the `bootstrap_map' structure up.  Don't use
> > +  /* Zero-initialize the `bootstrap_map' structure.  Don't use
> >       `memset' since it might not be built in or inlined and we cannot
> >       make function calls at this point.  Use '__builtin_memset' if we
> >       know it is available.  We do not have to clear the memory if we
> > @@ -365,12 +365,14 @@ _dl_start (void *arg)
> >       are initialized to zero by default.  */
> >  #ifndef DONT_USE_BOOTSTRAP_MAP
> >  # ifdef HAVE_BUILTIN_MEMSET
> > -  __builtin_memset (bootstrap_map.l_info, '\0', sizeof (bootstrap_map.l_info));
> > +  __builtin_memset (&bootstrap_map, '\0', sizeof (struct link_map));
> >  # else
> > -  for (size_t cnt = 0;
> > -       cnt < sizeof (bootstrap_map.l_info) / sizeof (bootstrap_map.l_info[0]);
> > -       ++cnt)
> > -    bootstrap_map.l_info[cnt] = 0;
> > +  {
> > +    char *p = (char *) &bootstrap_map;
> > +    char *pend = p + sizeof (struct link_map);
> > +    while (p < pend)
> > +      *(p++) = '\0';
> > +  }
> >  # endif
> >  #endif
> >
> > --
> > 2.7.4