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Date: Sat, 26 Sep 2026 13:52:35 +0000
Message-Id: <DLPAQECUALBD.2J6OV9FK2MA87@linux.dev>
Cc: <martin.lau@linux.dev>, <eddyz87@gmail.com>, <memxor@gmail.com>,
 <song@kernel.org>, <yonghong.song@linux.dev>, <jolsa@kernel.org>,
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 <mason@kernel.org>, <ihor.solodrai@linux.dev>
Subject: Re: [PATCH bpf v4 2/3] bpf: Skip detached progs in trampoline
 images that are still in use
From: "Florent Revest" <florent.revest@linux.dev>
To: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
 <bot+bpf-ci@kernel.org>, <bpf@vger.kernel.org>, <daniel@iogearbox.net>,
 <andrii@kernel.org>
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References: <20260925100342.481242-3-florent.revest@linux.dev>
 <6a6d43e6bada7a1433cd525c9ec31780ed9b2401c8834894defdf671601b2f1d@mail.kernel.org> <DLP46AE97EYD.1I2HDPGBR8GME@gmail.com>
In-Reply-To: <DLP46AE97EYD.1I2HDPGBR8GME@gmail.com>

On Sat Sep 26, 2026 at 8:44 AM UTC, Alexei Starovoitov wrote:
> On Fri, Sep 25, 2026 at 10:47 AM bot+bpf-ci@kernel.org <bot+bpf-ci@kernel=
.org> wrote:
> >> +	/* nops for move_imm+jirl, patched to skip this prog when it is deta=
ched */
> >> +	for (i =3D 0; i < LOONGARCH_LONG_JUMP_NINSNS; i++)
> >> +		emit_insn(ctx, nop);
> >
> > Can a task preempted inside the skip sled resume into half of the new j=
ump
> > sequence and jump to a garbage address?
>
> [...]
>
> > Could the LoongArch skip site use a single instruction instead? For
> > example, a 'b' to the skip target, which is always within the +/-128MB
> > range of the same image, patched from nop to 'b'. The rest of the sled
> > would then be unnecessary.
>
> Seems like a real bug. Same issue on riscv.
>
> ip_after_call was patched in bpf_tramp_image_put() when the image was
> already unreachable. Now bpf_trampoline_remove_prog() patches cur_image
> before bpf_trampoline_update() replaces it, so every call of the traced
> function goes through the sled while it is rewritten. stop_machine
> doesn't help the task that it preempted in the middle of the sled.
>
> Pls fix and respin.
> Everything else looks good.

Ok. In v5, on riscv and loongarch, it will emit single nops at skip sites a=
nd
patch them with jal or b instructions. They can always reach their target s=
ince
they are in the same image. This requires a new arch_bpf_trampoline_skip() =
hook
that defaults to bpf_arch_text_poke() on other archs. I tested it in QEMU.