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

Pedro Alves
2015-03-20T19:11:37Z

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