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From: Michael Ellerman <mpe@ellerman.id.au>
To: Ryan Sullivan <rysulliv@redhat.com>,
	live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Cc: rysulliv@redhat.com, joe.lawrence@redhat.com, pmladek@suse.com,
	mbenes@suse.cz, jikos@kernel.org, jpoimboe@kernel.org,
	naveen.n.rao@linux.ibm.com, christophe.leroy@csgroup.eu,
	npiggin@gmail.com
Subject: Re: [PATCH] powerpc/ftrace: restore r2 to caller's stack on livepatch sibling call
Date: Tue, 30 Jul 2024 00:17:18 +1000	[thread overview]
Message-ID: <878qxkp9jl.fsf@mail.lhotse> (raw)
In-Reply-To: <20240724183321.9195-1-rysulliv@redhat.com>

Hi Ryan,

Thanks for the patch.

Ryan Sullivan <rysulliv@redhat.com> writes:
> Currently, on PowerPC machines, sibling calls in livepatched functions
> cause the stack to be corrupted and are thus not supported by tools
> such as kpatch. Below is an example stack frame showing one such
> currupted stacks:
...
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index 76dbe9fd2c0f..4dfbe6076ad1 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -244,6 +244,9 @@ livepatch_handler:
>  	mtlr	r12
>  	ld	r2,  -24(r11)
>  
> +	/* Restore toc to caller's stack in case of sibling call */
> +	std	r2, 24(r1)
> +

It would be good to have a comment here explaining why it's safe in all
cases to store the current r2 value back to the caller's save slot.

I haven't convinced myself that it is always safe, but I need to think
about it a bit harder O_o

cheers

WARNING: multiple messages have this Message-ID (diff)
From: Michael Ellerman <mpe@ellerman.id.au>
To: Ryan Sullivan <rysulliv@redhat.com>,
	live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Cc: pmladek@suse.com, joe.lawrence@redhat.com, jikos@kernel.org,
	christophe.leroy@csgroup.eu, rysulliv@redhat.com,
	npiggin@gmail.com, naveen.n.rao@linux.ibm.com, mbenes@suse.cz,
	jpoimboe@kernel.org
Subject: Re: [PATCH] powerpc/ftrace: restore r2 to caller's stack on livepatch sibling call
Date: Tue, 30 Jul 2024 00:17:18 +1000	[thread overview]
Message-ID: <878qxkp9jl.fsf@mail.lhotse> (raw)
In-Reply-To: <20240724183321.9195-1-rysulliv@redhat.com>

Hi Ryan,

Thanks for the patch.

Ryan Sullivan <rysulliv@redhat.com> writes:
> Currently, on PowerPC machines, sibling calls in livepatched functions
> cause the stack to be corrupted and are thus not supported by tools
> such as kpatch. Below is an example stack frame showing one such
> currupted stacks:
...
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index 76dbe9fd2c0f..4dfbe6076ad1 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -244,6 +244,9 @@ livepatch_handler:
>  	mtlr	r12
>  	ld	r2,  -24(r11)
>  
> +	/* Restore toc to caller's stack in case of sibling call */
> +	std	r2, 24(r1)
> +

It would be good to have a comment here explaining why it's safe in all
cases to store the current r2 value back to the caller's save slot.

I haven't convinced myself that it is always safe, but I need to think
about it a bit harder O_o

cheers

  reply	other threads:[~2024-07-29 14:17 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-07-24 18:33 [PATCH] powerpc/ftrace: restore r2 to caller's stack on livepatch sibling call Ryan Sullivan
2024-07-24 18:33 ` Ryan Sullivan
2024-07-29 14:17 ` Michael Ellerman [this message]
2024-07-29 14:17   ` Michael Ellerman
2024-07-29 15:02   ` Ryan Sullivan
2024-07-29 15:02     ` Ryan Sullivan
2024-08-15 10:22     ` Michael Ellerman
2024-08-15 16:07       ` Ryan Sullivan
2024-08-15 16:24         ` Joe Lawrence
2024-09-09 16:33           ` Ryan B. Sullivan
2024-09-10  7:21             ` Michael Ellerman
2024-09-10 15:02               ` Ryan B. Sullivan
2024-08-08  7:00 ` Christophe Leroy
2024-08-08  7:00   ` Christophe Leroy
2024-08-08 17:55   ` [PATCH v2] powerpc/ftrace: restore caller's toc on ppc64 " Ryan Sullivan
2024-08-08 17:55     ` Ryan Sullivan
2024-08-08 21:17   ` Ryan Sullivan
2024-08-08 21:17     ` Ryan Sullivan

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