linux-kernel - Re: [PATCH v2 6/7] ptrace: introduce PTRACE_SET_SYSCALL

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Message-ID: <20250117153258.GC21203@redhat.com>
Date: Fri, 17 Jan 2025 16:32:59 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Dmitry V. Levin" <ldv@...ace.io>
Cc: Eugene Syromyatnikov <evgsyr@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	Renzo Davoli <renzo@...unibo.it>,
	Davide Berardi <berardi.dav@...il.com>,
	strace-devel@...ts.strace.io, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v2 6/7] ptrace: introduce PTRACE_SET_SYSCALL_INFO request

Dmitry,

You certainly understand the user-space needs much better than me.
I am just trying to understand your point.

On 01/17, Dmitry V. Levin wrote:
>
> We should accept larger user_size from the very beginning, so that in case
> the structure grows in the future, the userspace that sicks to the current
> set of supported features would be still able to work with older kernels.

This is what I can't understand, perhaps I have a blind spot here ;)

Could you provide an example (even absolutely artificial) of possible extension
which can help me to understand?

> We cannot just use sizeof(info) because it depends on the alignment of
> __u64.

Hmm why? I thought that the kernel already depends on the "natural" alignment?
And if we can't, then PTRACE_SYSCALL_INFO_SIZE_VER0 added by this patch makes
no sense?

Sorry I guess I must have missed something, I am sick today.

> Also, I don't think we need to fill with zeroes the trailing
> padding bytes of the structure as we are not going to use them in any way.

At least we seem to agree here ;)

Oleg.