linux-kernel - Re: [PATCH bpf-next 0/2] Implement BPF formatted output helpers with bstr

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Date:   Fri, 23 Apr 2021 10:50:22 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Florent Revest <revest@...omium.org>, bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kpsingh@...nel.org, jackmanb@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 0/2] Implement BPF formatted output helpers with
 bstr_printf

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).

Rasmus