[Bug dynamic-link/30100] [patch] add dlmem()

["stsp at users dot sourceforge.net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30100

--- Comment #12 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Adhemerval Zanella from comment #10)
> The filebuf
> struct is already a static scratch buffer used exactly to avoid realloc, why
> exactly do you need a value larger than that?

It is used in glibc only for Ehdr.
Then both open_verify() and _dl_map_object_from_fd() have this
(duplicated) piece of code to read the rest:

  maplength = header->e_phnum * sizeof (ElfW(Phdr));
  if (header->e_phoff + maplength <= (size_t) fbp->len)
    phdr = (void *) (fbp->buf + header->e_phoff);
  else
    {
      phdr = alloca (maplength);
      if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength,
                                       header->e_phoff) != maplength)
        {
          errstring = N_("cannot read file data");
          goto lose_errno;
        }
    }

Because fbp only stores Ehdr, that condition always goes the
second branch, and reads the rest. It does so in both
open_verify() and _dl_map_object_from_fd().
I wanted to share/re-use most of the _dl_map_object_from_fd()
code, but for that I had to unbind it from "fd" as much as possible.
So I decided to de-duplicate the aforementioned piece and allow
it to read all elf headers to filebuf, rather than twice reading
into an alloca() space as in the aforementioned fragment.

But now I do that w/o reallocating because I can defer the allocation
until the header size is known.

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