Re: [PATCH 1/4] Improve identification of memory mappings

[Pedro Alves <palves@redhat.com>]

On 03/19/2015 11:06 PM, Sergio Durigan Junior wrote:

> However, IMHO gcore_create_callback still has some problems.  For
> example, this heuristic used to determine whether a mapping should be
> dumped or not:
> 
>   if (write == 0 && modified == 0 && !solib_keep_data_in_core (vaddr, size))
>     {
>       /* See if this region of memory lies inside a known file on disk.
> 	 If so, we can avoid copying its contents by clearing SEC_LOAD.  */
>       struct objfile *objfile;
>       struct obj_section *objsec;
> 
>       ALL_OBJSECTIONS (objfile, objsec)
> 	{
> 	  bfd *abfd = objfile->obfd;
> 	  asection *asec = objsec->the_bfd_section;
> 	  bfd_vma align = (bfd_vma) 1 << bfd_get_section_alignment (abfd,
> 								    asec);
> 	  bfd_vma start = obj_section_addr (objsec) & -align;
> 	  bfd_vma end = (obj_section_endaddr (objsec) + align - 1) & -align;
> 
> 	  /* Match if either the entire memory region lies inside the
> 	     section (i.e. a mapping covering some pages of a large
> 	     segment) or the entire section lies inside the memory region
> 	     (i.e. a mapping covering multiple small sections).
> 
> 	     This BFD was synthesized from reading target memory,
> 	     we don't want to omit that.  */
> 	  if (objfile->separate_debug_objfile_backlink == NULL
> 	      && ((vaddr >= start && vaddr + size <= end)
> 	          || (start >= vaddr && end <= vaddr + size))
> 	      && !(bfd_get_file_flags (abfd) & BFD_IN_MEMORY))
> 	    {
> 	      flags &= ~(SEC_LOAD | SEC_HAS_CONTENTS);
> 	      goto keep;	/* Break out of two nested for loops.  */
> 	    }
> 	}
> 
>     keep:;
>     }
> 
> will not be used by any code, because everyone will be passing
> 'modified' as 1 with my following patch (the only code that could pass
> 'modified' as zero was linux_find_memory_regions_full, which I will
> patch to only pass 1 as well).

Alright.  Good that that now became clear.

I found the initial submission for that, btw:

  https://sourceware.org/ml/gdb-patches/2003-10/msg00164.html

I wonder whether it'd be worth to keep that somehow, for the fallback
cases when /proc//smaps or some other /proc file you're relying
on for file-backed read-only region identification is missing
(because old kernel, or even /proc not mounted).  Maybe not.

Thanks,
Pedro Alves