[Bug 1908063] Re: gdb-multiarch 8.1.1-0ubuntu1 regression: assertion failure or internal error when connecting to QEMU aarch64 gdbstub

[Peter Maydell]

It looks like the reason QEMU's test suite passed was that the older
Ubuntu gdb didn't have a fix for LP:1901026 (support remote connection
over UNIX domain socket), so the test suite would simply skip the
offending test and never get as far as falling over the assertion
failure. After pulling various old binaries out of my backups to test
I'm forced to conclude that I was simply misremembering by-hand use of
the gdbstub working and I must not have used it as recently as I'd
thought. So this is a bug, but not a regression.

A possible set of commits for a cherry-picked fix would be
c8ec2f334c3751c28d5f and 4ef0bef68cfc777e59156fce1 (which together
remove the hard-coded limit on register size and thus also the
assertion), but the former is not a completely trivial patch. I haven't
attempted such a cherry-pick to see if it works.

A workaround is to tell QEMU not to emulate SVE (eg with "-cpu
max,sve=off" -- it is the large SVE vector registers which trigger the
problem.


** Summary changed:

- gdb-multiarch 8.1.1-0ubuntu1 regression: assertion failure or internal error when connecting to QEMU aarch64 gdbstub
+ gdb-multiarch 8.1.1-0ubuntu1: assertion failure or internal error when connecting to QEMU aarch64 gdbstub

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Title:
  gdb-multiarch 8.1.1-0ubuntu1: assertion failure or internal error when
  connecting to QEMU aarch64 gdbstub

Status in gdb package in Ubuntu:
  New
Status in gdb source package in Bionic:
  New

Bug description:
  This bug is a regression introduced in 8.1.1-0ubuntu1 for Bionic --
  the previous 8.1-0ubuntu3.2 gdb works fine with QEMU's gdbstub.

  Reproduce:
  Get the sources for QEMU 5.2.0, and build the aarch64-linux-user target. (It looks like Bionic's QEMU is old enough that it doesn't provoke this gdb error.)

  Run
   qemu-aarch64 -g 1234 path/to/some/aarch64/binary

  In another terminal, start gdb-multiarch. At the (gdb) prompt, type
  "target remote :1234". GDB will dump core:

  (gdb) target remote :1234
  Remote debugging using :1234
  Recursive internal problem.
  Aborted (core dumped)

  You can see a variant of this also in the QEMU test suite with "make
  -C your-qemu-build-dir check-tcg", where the error message includes an
  assertion failure:

  [...]

  timeout 60  /home/petmay01/linaro/qemu-from-laptop/qemu/tests/guest-debug/run-test.py --gdb /usr/bin/gdb-multiarch --qemu /home/petmay01/linaro/qemu-from-laptop/qemu/build/aarch64-linux/qemu-aarch64 --qargs "" --bin sha1 --test /home/petmay01/linaro/qemu-from-laptop/qemu/tests/tcg/multiarch/gdbstub/sha1.py >  run-gdbstub-sha1.out
  /build/gdb-veKdC1/gdb-8.1.1/gdb/regcache.c:122: internal-error: void* init_regcache_descr(gdbarch*): Assertion `MAX_REGISTER_SIZE >= descr->sizeof_register[i]' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.

  This is a bug, please report it.  For instructions, see:
  <http://www.gnu.org/software/gdb/bugs/>.

  Aborted (core dumped)
  /home/petmay01/linaro/qemu-from-laptop/qemu/tests/tcg/multiarch/Makefile.target:51: recipe for target 'run-gdbstub-sha1' failed

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