[Bug gdb/30525] gdb cannot read shared libraries on SPARC64

["pedro at palves dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30525

--- Comment #3 from Pedro Alves <pedro at palves dot net> ---
> (gdb) info sharedlibrary
> Cannot access memory at address 0xfff80001002011e0
> Cannot access memory at address 0xfff80001002011d8
> Cannot access memory at address 0xfff80001002011d8

Those addresses look suspiciously high for userspace.  They are 64-bit
addresses, with the high bits set.  When interpreted as signed, they'd be
negative.  Are they really correct?

We have, in gdb/linux-nat.c:

static enum target_xfer_status
linux_proc_xfer_memory_partial_fd (int fd, int pid,
                                   gdb_byte *readbuf, const gdb_byte *writebuf,
                                   ULONGEST offset, LONGEST len,
                                   ULONGEST *xfered_len)
{
  ssize_t ret;

  gdb_assert (fd != -1);

  /* Use pread64/pwrite64 if available, since they save a syscall and can
     handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
     debugging a SPARC64 application).  */
#ifdef HAVE_PREAD64
  ret = (readbuf ? pread64 (fd, readbuf, len, offset)
         : pwrite64 (fd, writebuf, len, offset));
#else
  ret = lseek (fd, offset, SEEK_SET);
  if (ret != -1)
    ret = (readbuf ? read (fd, readbuf, len)
           : write (fd, writebuf, len));
#endif


So one thing to check is whether the HAVE_PREAD64 code is enabled.  And then,
that may be failing due to a negative offset, give pread64's offset parameter
is an off_t.

Try enabling "set debug linux-nat 1", so that the following log is output:

  if (ret == -1)
    {
      linux_nat_debug_printf ("accessing fd %d for pid %d failed: %s (%d)",
                              fd, pid, safe_strerror (errno), errno);
      return TARGET_XFER_E_IO;
    }

Maybe the errno gives us some clue.

Does hacking the linux-nat.c:proc_mem_file_is_writable to always return false
paper over the problem?

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