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ARM: 9490/1: backtrace-clang: fix wrong sp usage for unwinding

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show_stack() can be called for any task, but c_backtrace is always
unwinding frames based on "sp" register.

Which leads to printing backtrace of current task instead of
required task.

Try with normal TC:
==================
       for_each_process(p){
               sched_show_task(p);
       }

[7.433271] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
[7.433633] Call trace:
[7.433640] [<80113418>] (dump_backtrace) from [<80113510>] (show_stack+0x14/0x18)
..
[7.433676] [<8016749c>] (sched_show_task) from [<803cb324>] (meminfo_proc_show+0x6c/0x930)
[7.434019]  r5:8158e300 r4:8b4882d0
[7.434024] [<803cb324>] (meminfo_proc_show) from [<80365788>] (seq_read_iter+0x148/0x4bc)
[7.434045] [<80365788>] (seq_read_iter) from [<803c044c>] (proc_reg_read_iter+0xb8/0xc4)
[7.434060] [<803c044c>] (proc_reg_read_iter) from [<80377104>] (copy_splice_read+0x228/0x308)

It should unwind frame based on passed "fp".
(CONFIG_UNWINDER_FRAME_POINTER=y)

Extra sp manipulation is needed only when c_backtrace()
is called from dump_backtrace() with "frame == fp" i.e.
NULL regs and not current task. (Suggested by Ard Biesheuvel)

Otherwise passed "fp" should be used to unwind backtraces.

With fix:
=========
[13.933732] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
[13.934165] Call trace:
[13.934604] [<80afa1a8>] (schedule) from [<8015426c>] (kthreadd+0x124/0x208)
[13.934654]  r10:8100bbf0 r4:8116f440
[13.934664] [<8015426c>] (kthreadd) from [<8010010c>] (ret_from_fork+0x14/0x28)
[13.934691] Exception stack(0xf081df9c to 0xf081dfe4)

Suggested-by:  Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Onkarnath <onkarnath.1@samsung.com>
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Onkarnath 2026-08-11 10:52:19 +01:00 committed by Russell King
parent f8ebc827f2
commit 358515f9f9
1 changed files with 3 additions and 1 deletions
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4
arch/arm/lib/backtrace-clang.S
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@ -107,8 +107,10 @@ ENDPROC(c_backtrace)
/*
* Switches the current frame to be the frame for dump_stack.
* (Only in case of current task using current registers)
*/
add frame, sp, #24 @ switch to false frame
cmp frame, fp
addeq frame, sp, #24 @ switch to false frame
for_each_frame: tst frame, mask @ Check for address exceptions
bne no_frame