[Bug dynamic-link/26322] New: Libraries may fail to load if (p)read doesn't return all bytes requested

["jasonwhite at fb dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26322

            Bug ID: 26322
           Summary: Libraries may fail to load if (p)read doesn't return
                    all bytes requested
           Product: glibc
           Version: 2.33
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: jasonwhite at fb dot com
  Target Milestone: ---

In `elf/dl-load.c` there is a call to pread[1]:

    if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength,
                                    ehdr->e_phoff) != maplength)
      {
      read_error:
          errval = errno;
          errstring = N_("cannot read file data");
          goto call_lose;
      }

However, there is no guarantee that pread will return all the bytes requested.
Thus, I believe the read should be wrapped in a loop to ensure it receives all
the bytes requested.

I found this issue while simulating a pathological kernel that only returns one
byte at a time for read calls using ptrace. I imagine this could also be
reproduced by writing a small FUSE file system that only returns a byte at a
time for reads.

[1]:
https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/dl-load.c;h=c1bc8c26be1229fafc2a03afd9fb27edbcf8c3a8;hb=95c1056962a3f2297c94ce47f0eaf0c5b6563231#l1667

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