Re: [PATCH bpf 2/7] bpf/x86: prevent trampoline attachment when args location on stack is uncertain

From: "Alexis Lothoré" <alexis.lothore@bootlin.com>
To: "Peter Zijlstra" <peterz@infradead.org>
Cc: "Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
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Subject: Re: [PATCH bpf 2/7] bpf/x86: prevent trampoline attachment when args location on stack is uncertain
Date: Fri, 13 Jun 2025 10:59:28 +0200	[thread overview]
Message-ID: <DALA5WYA04OG.1283TZDOVLBPS@bootlin.com> (raw)
In-Reply-To: <20250613083232.GL2273038@noisy.programming.kicks-ass.net>

On Fri Jun 13, 2025 at 10:32 AM CEST, Peter Zijlstra wrote:
> On Fri, Jun 13, 2025 at 10:26:37AM +0200, Alexis Lothoré wrote:
>> Hi Peter,
>> 
>> On Fri Jun 13, 2025 at 10:11 AM CEST, Peter Zijlstra wrote:
>> > On Fri, Jun 13, 2025 at 09:37:11AM +0200, Alexis Lothoré (eBPF Foundation) wrote:

[...]

>> Maybe my commit wording is not precise enough, but indeed, there's not
>> doubt about whether the struct value is passed on the stack or through a
>> register/a pair of registers. The doubt is rather about the struct location
>> when it is passed _by value_ and _on the stack_: the ABI indeed clearly
>> states that "Structures and unions assume the alignment of their most
>> strictly aligned component" (p.13), but this rule is "silently broken" when
>> a struct has an __attribute__((packed)) or and __attribute__((aligned(X))),
>> and AFAICT this case can not be detected at runtime with current BTF info.
>
> Ah, okay. So it is a failure of BTF. That was indeed not clear.

If I need to respin, I'll rewrite the commit message to include the details
above.

Alexis

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com