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From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Florent Revest <revest@chromium.org>, bpf@vger.kernel.org
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
	kpsingh@kernel.org, jackmanb@google.com,
Subject: Re: [PATCH bpf-next 0/2] Implement BPF formatted output helpers with bstr_printf
Date: Fri, 23 Apr 2021 10:50:22 +0200	[thread overview]
Message-ID: <8f89faf1-d7e6-ebe0-fb7d-c5b8243d140a@rasmusvillemoes.dk> (raw)
In-Reply-To: <20210423011517.4069221-1-revest@chromium.org>

On 23/04/2021 03.15, Florent Revest wrote:
> Our formatted output helpers are currently implemented with
> snprintf-like functions which take arguments as va_list but the types
> stored in a va_list need to be known at compilation time which causes
> problems when dealing with arguments from the BPF world that are always
> u64 but considered differently depending on the format specifiers they
> are associated with at runtime.
> This series replaces snprintf usages with bstr_printf calls. This lets
> us construct a binary representation of arguments in bpf_printf_prepare
> at runtime that matches an ABI that is neither arch nor compiler
> specific.
> This solves a bug reported by Rasmus Villemoes that would mangle
> arguments on 32 bit machines.

That's not entirely accurate. The arguments are also mangled on x86-64,
it's just that in a few cases that goes unnoticed. That's why I
suggested you try and take your test case (which I assume had been
passing with flying colours on x86-64) and rearrange the specifiers,
arguments and expected output string so that the (morally) 32 bit
arguments end up beyond those-that-end-up-in-the-reg_save_area.

IOWs, it is the 32 bit arguments that are mangled (because they get
passed as-if they were actually 64 bits), and that applies on all
architectures; nothing to do with sizeof(long).


  parent reply	other threads:[~2021-04-23  8:50 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-23  1:15 [PATCH bpf-next 0/2] Implement BPF formatted output helpers with bstr_printf Florent Revest
2021-04-23  1:15 ` [PATCH bpf-next 1/2] seq_file: Add a seq_bprintf function Florent Revest
2021-04-23  1:15 ` [PATCH bpf-next 2/2] bpf: Implement formatted output helpers with bstr_printf Florent Revest
2021-04-23  9:27   ` Rasmus Villemoes
2021-04-23 13:45     ` Florent Revest
2021-04-23  8:50 ` Rasmus Villemoes [this message]
2021-04-23 13:26   ` [PATCH bpf-next 0/2] Implement BPF " Florent Revest
2021-04-23 14:31     ` Rasmus Villemoes

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