[Bug gdb/26196] New: gdb error message is unclear when attempting to load a file >2GB via gdbserver

["david.spickett at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26196

            Bug ID: 26196
           Summary: gdb error message is unclear when attempting to load a
                    file >2GB via gdbserver
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: david.spickett at linaro dot org
  Target Milestone: ---

This is related to https://sourceware.org/bugzilla/show_bug.cgi?id=23198, filed
as a new ticket for the gdb error message specifically.

I attempted to debug lldb by attaching gdbserver to it then connecting to that
with gdb. lldb references a shared library that contains its commands. This
library is >2GB in size when built in debug mode.

The error message gdb gives me when trying to load this is:
Reading
/work/open_source/lldb-cross-compile/build-host-debug/bin/../lib/liblldb.so.11git
from remote target...
Error while mapping shared library sections:
`target:/work/open_source/lldb-cross-compile/build-host-debug/bin/../lib/liblldb.so.11git':
not in executable format: file format not recognized

(gdb) info shared
>From                To                  Syms Read   Shared Object Library
<...>
                                        No         
/work/open_source/lldb-cross-compile/build-host-debug/bin/../lib/liblldb.so.11git

We can see from logging in gdbserver that the failure is because the offset
sent to pread is invalid when trying to get the first section header.
require_int in gdbserver is checking that the offset is not > 0x80000000.
getpkt ("vFile:pread:a,3fff,9ab473c8");  [no ack sent]
putpkt ("$F-1,16#37"); [noack mode] // 0x16 aka FILEIO_EINVAL

(https://sourceware.org/gdb/current/onlinedocs/gdb/Host-I_002fO-Packets.html)

The error message implies that the format is incorrect (although it earlier
read the ELF bytes) and I initially thought that gdbserver had read the symlink
as a real file (liblldb.so.11git links to the real .so). Which I'm not sure is
even possible but I tried to narrow it down from there.

I think this error message should say something like "<file>: file offset of X
is beyond limit of vFile:pread packet". (or bail a lot earlier once we see that
the size is >2GB)
The root issue would be the vFile packet itself but this is a niche use case
and you can workaround it by setting a sysroot to load the file from the local
machine. An improved error would make it clearer what you need to do.

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