[PATCH] powerpc64/ftrace: fix clobbered r15 during livepatching

[Naveen N Rao]

On Thu, Apr 17, 2025 at 12:42:27AM +0530, Hari Bathini wrote:
> While r15 is clobbered always with PPC_FTRACE_OUT_OF_LINE, it is
> not restored in livepatch sequence leading to not so obvious fails
> like below:
> 
>   BUG: Unable to handle kernel data access on write at 0xc0000000000f9078
>   Faulting instruction address: 0xc0000000018ff958
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   ...
>   NIP:  c0000000018ff958 LR: c0000000018ff930 CTR: c0000000009c0790
>   REGS: c00000005f2e7790 TRAP: 0300   Tainted: G              K      (6.14.0+)
>   MSR:  8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 2822880b  XER: 20040000
>   CFAR: c0000000008addc0 DAR: c0000000000f9078 DSISR: 0a000000 IRQMASK: 1
>   GPR00: c0000000018f2584 c00000005f2e7a30 c00000000280a900 c000000017ffa488
>   GPR04: 0000000000000008 0000000000000000 c0000000018f24fc 000000000000000d
>   GPR08: fffffffffffe0000 000000000000000d 0000000000000000 0000000000008000
>   GPR12: c0000000009c0790 c000000017ffa480 c00000005f2e7c78 c0000000000f9070
>   GPR16: c00000005f2e7c90 0000000000000000 0000000000000000 0000000000000000
>   GPR20: 0000000000000000 c00000005f3efa80 c00000005f2e7c60 c00000005f2e7c88
>   GPR24: c00000005f2e7c60 0000000000000001 c0000000000f9078 0000000000000000
>   GPR28: 00007fff97960000 c000000017ffa480 0000000000000000 c0000000000f9078
>   ...
>   Call Trace:
>     check_heap_object+0x34/0x390 (unreliable)
>   __mutex_unlock_slowpath.isra.0+0xe4/0x230
>   seq_read_iter+0x430/0xa90
>   proc_reg_read_iter+0xa4/0x200
>   vfs_read+0x41c/0x510
>   ksys_read+0xa4/0x190
>   system_call_exception+0x1d0/0x440
>   system_call_vectored_common+0x15c/0x2ec
> 
> Fix it by restoring r15 always.
> 
> Fixes: eec37961a56a ("powerpc64/ftrace: Move ftrace sequence out of line")
> Reported-by: Viktor Malik <vmalik at redhat.com>
> Closes: https://lore.kernel.org/lkml/1aec4a9a-a30b-43fd-b303-7a351caeccb7@redhat.com
> Cc: stable at vger.kernel.org # v6.13+
> Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
> ---
>  arch/powerpc/kernel/trace/ftrace_entry.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index 2c1b24100eca..3565c67fc638 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -212,10 +212,10 @@
>  	bne-	1f
>  
>  	mr	r3, r15
> +1:	mtlr	r3
>  	.if \allregs == 0
>  	REST_GPR(15, r1)
>  	.endif
> -1:	mtlr	r3
>  #endif

LGTM.
Acked-by: Naveen N Rao (AMD) <naveen at kernel.org>


- Naveen