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From: "idan.horowitz at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug tdep/28796] New: GDB locks up after stopping on a breakpoint in
 invalidated memory on aarch64
Date: Thu, 20 Jan 2022 09:41:52 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D28796

            Bug ID: 28796
           Summary: GDB locks up after stopping on a breakpoint in
                    invalidated memory on aarch64
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: idan.horowitz at gmail dot com
  Target Milestone: ---

When debugging a kernel inside QEMU aarch64-softmmu using GDB for
aarch64-none-linux-gnu, when GDB pauses execution after a breakpoint is hit=
 in
invalidated memory (that is, memory that does not have a valid translation
yet), GDB locks up.
Meaning that execution can not be resumed again (using stepi/continue/advan=
ce
etc), this is due to the fact that GDB tries to read the memory at the curr=
ent
PC when resuming execution, which it fails to do due to the invalid
translation, but this also stops it from stepping into the translation
exception handler, essentially locking up GDB completely.
I've tracked down said memory read to the following call:
https://sourceware.org/git/?p=3Dbinutils-gdb.git;a=3Dblob;f=3Dgdb/aarch64-t=
dep.c;h=3D63d626f90ace55f80c5d3e3d557993d905a90da1;hb=3DHEAD#l2869
This read is used to detect and execute load-exclusive/store-exclusive
sequences atomically, but the issue is the fact that this method uses the
unsafe read_memory_unsigned_integer method, which throws when a read fails,
instead of calling safe_read_memory_unsigned_integer and returning an empty
vector like the rest of the failure paths in this method do. This exception
then bubbles up to `resume_1` which cancels the resumption of the execution,
locking GDB in place.

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