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* [Bug backtrace/27987] New: aarch64 prologue unwinder does not correctly handle -fpatchable-function-entry=2 flag
@ 2021-06-16 15:39 ctice42 at gmail dot com
  2021-06-16 15:44 ` [Bug backtrace/27987] " ctice42 at gmail dot com
                   ` (7 more replies)
  0 siblings, 8 replies; 9+ messages in thread
From: ctice42 at gmail dot com @ 2021-06-16 15:39 UTC (permalink / raw)
  To: gdb-prs

https://sourceware.org/bugzilla/show_bug.cgi?id=27987

            Bug ID: 27987
           Summary: aarch64 prologue unwinder does not correctly handle
                    -fpatchable-function-entry=2 flag
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: backtrace
          Assignee: unassigned at sourceware dot org
          Reporter: ctice42 at gmail dot com
  Target Milestone: ---

While debugging kgdb on a linux kernel built for aarch64 (inside chromiumos),
we found that if we built the kernel with -fpatchable-function-entry=2 and
tried to do a backtrace, the aarch64 prologue unwinder would not properly
handle/ignore the nops at the start of the function prologue, and would then
get confused about the stack, and would hit an assertion failure and crash gdb:

~/trunk/src/third_party/kernel/v5.4 $ aarch64-cros-linux-gnu-gdb         
vmlinux          -ex "target remote localhost:1234"
GNU gdb (Chromium OS 9.2.20200923 vanilla) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
…
Reading symbols from vmlinux...
Remote debugging using localhost:1234
arch_kgdb_breakpoint ()
    at
/home/cmtice/trunk/src/third_party/kernel/v5.4/./arch/arm64/include/asm/kgdb.h:21
21              asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
(gdb) bt
#0  arch_kgdb_breakpoint ()
    at
/home/cmtice/trunk/src/third_party/kernel/v5.4/./arch/arm64/include/asm/kgdb.h:21
#1  kgdb_breakpoint () at kernel/debug/debug_core.c:1208
/var/tmp/portage/cross-aarch64-cros-linux-gnu/gdb-9.2.20200923-r5/work/gdb-9.2/gdb/inline-frame.c:155:
internal-error: void inline_frame_this_id(struct frame_info *, void **, struct
frame_id *): Assertion `frame_id_p (*this_id)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) y



If we build the kernel WITHOUT the -fpatchable-function-entry=2 flag and do the
same backtrace as the same location, we do get a corrupted stack error message,
but we don't hit the assert or the crash, we get a different (more correct)
backtrace, and because GDB doesn't actually crash, we can continue to do some
useful debugging:

~/trunk/src/third_party/kernel/v5.4 $ aarch64-cros-linux-gnu-gdb         
vmlinux          -ex "target remote localhost:1234"
GNU gdb (Chromium OS 9.2.20200923 vanilla) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
…
Reading symbols from vmlinux...
Remote debugging using localhost:1234
arch_kgdb_breakpoint ()
    at
/home/cmtice/trunk/src/third_party/kernel/v5.4/./arch/arm64/include/asm/kgdb.h:21
21              asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
(gdb) bt
#0  arch_kgdb_breakpoint ()
    at
/home/cmtice/trunk/src/third_party/kernel/v5.4/./arch/arm64/include/asm/kgdb.h:21
#1  kgdb_breakpoint () at kernel/debug/debug_core.c:1208
#2  0xffff8000131d0cd0 in kgdb_initial_breakpoint ()
    at kernel/debug/debug_core.c:1011
#3  dbg_late_init () at kernel/debug/debug_core.c:1026
#4  0xffff8000131b0bc4 in start_kernel () at init/main.c:1044
#5  0x0000000000000000 in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) 


Sadly, the only test case I have been able to find is complex to reproduce, as
it
involves downloading and setting up a chromiumos chroot, then downloading,
configuring and building a copy of the linux kernel, then running the kernel in
qemu in the chromiumos chroot and running gdb in a separate terminal window. 
If someone really wants/needs the complete set of reproduction steps, let me
know and I will supply them.

I did debug this issue enough to verify that it is indeed the aarch64 prologue
unwinder that has the issue, but my knowledge of aarch64 is not good enough to
allow me to fix the issue.

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^ permalink raw reply	[flat|nested] 9+ messages in thread
end of thread, other threads:[~2021-06-29 17:08 UTC | newest]

Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-06-16 15:39 [Bug backtrace/27987] New: aarch64 prologue unwinder does not correctly handle -fpatchable-function-entry=2 flag ctice42 at gmail dot com
2021-06-16 15:44 ` [Bug backtrace/27987] " ctice42 at gmail dot com
2021-06-25 13:01 ` luis.machado at linaro dot org
2021-06-26  6:28 ` ctice42 at gmail dot com
2021-06-28 17:52 ` luis.machado at linaro dot org
2021-06-29 14:00 ` luis.machado at linaro dot org
2021-06-29 14:34 ` luis.machado at linaro dot org
2021-06-29 17:07 ` luis.machado at linaro dot org
2021-06-29 17:08 ` luis.machado at linaro dot org

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