Re: Silence gcc-8 warnings

[Michael Matz <matz at suse dot de>]

Hi,

On Tue, 24 Apr 2018, Alan Modra wrote:

> On Tue, Apr 24, 2018 at 12:41:31AM +0930, Alan Modra wrote:
> > Yeah, really curious.  I failed to mention that gcc build was on
> > hppa-linux, whereas the one that didn't show any error was on
> > x86_64-linux.  I have a little sleuthing to do to figure out what made
> > the difference.  It doesn't seem likely that it was any of the more
> > recent gcc patches.
> 
> Oh wow, I don't see the first error on x86_64-linux (for
> swap_linux_prpsinfo32_ugid32_out) when the preprocessed source looks
> like the following.  However, take out the file/line directives and
> the problem appears!

The elf-linux-core.h header is regarded as system header with the #line 
directives, and that disables the warning.  I've looked at this somewhat 
further, and actually the warning is correct (!), but subtle:

> static inline void
> swap_linux_prpsinfo32_ugid32_out
>   (bfd *obfd,
>    const struct elf_internal_linux_prpsinfo *from,
>    struct elf_external_linux_prpsinfo32_ugid32 *to)

So, FROM has different type from TO (xxx vs xxx_ugid32):
from (from elf-bfd.h):
  struct elf_internal_linux_prpsinfo
  {
    ...
    char pr_fname[16 + 1];
    char pr_psargs[80 + 1];
  };
and to (from elf-linux-core.h):
struct elf_external_linux_prpsinfo32_ugid32
  {
    ...
    char pr_fname[16];
    char pr_psargs[80];
  };

So, the strncpy was:

 __builtin_strncpy (to->pr_fname, from->pr_fname, sizeof (to->pr_fname))

sizeof(to->prfname) is 16, but sizeof(from->prfname) is 17, so it is 
indeed conceivable that the from string has 16 characters plus null 
terminator, which would not fit terminated into to->pr_fname, and this is 
what is warned about.  I think this is a genuine bug in bfd (even though 
possibly a harmless one with non-fuzzed prpsinfo structs in core files).


Ciao,
Michael.