linux-kernel - Re: [PATCH v2] docs/bpf: clarify ret handling in LSM BPF programs

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Message-ID: <CACKMdfkPsemrUMPXNO5OR9Y2y70xNVVY9ss-3hX9NtGXFJxyQg@mail.gmail.com>
Date: Thu, 18 Sep 2025 13:03:40 +0300
From: Ariel Silver <arielsilver77@...il.com>
To: KP Singh <kpsingh@...nel.org>
Cc: bpf@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, mattbobrowski@...gle.com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, sdf@...ichev.me, haoluo@...gle.com, 
	jolsa@...nel.org, corbet@....net
Subject: Re: [PATCH v2] docs/bpf: clarify ret handling in LSM BPF programs

Just so I understand.
1) The docs are indeed wrong, correct?
2) Any idea why I get an automatic response of "CONFLICTS" when I send
my patch? Even though im 1000% sure there are no conlicts

On Wed, Sep 17, 2025 at 12:31 PM KP Singh <kpsingh@...nel.org> wrote:
>
> On Thu, Sep 11, 2025 at 12:52 PM Ariel Silver <arielsilver77@...il.com> wrote:
> >
> > v2: Fixed trailing whitespace (reported by checkpatch.pl)
> >
> > Docs currently suggest that all attached BPF LSM programs always run
> > and that ret simply carries the previous return code. In reality,
> > execution stops as soon as one program returns non-zero. This is
> > because call_int_hook() breaks out of the loop when RC != 0, so later
> > programs are not executed.
> >
> > Signed-off-by: arielsilver77@...il.com <arielsilver77@...il.com>
> > ---
> >  Documentation/bpf/prog_lsm.rst | 12 ++++--------
> >  1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/bpf/prog_lsm.rst b/Documentation/bpf/prog_lsm.rst
> > index ad2be02f3..92bfb64c2 100644
> > --- a/Documentation/bpf/prog_lsm.rst
> > +++ b/Documentation/bpf/prog_lsm.rst
> > @@ -66,21 +66,17 @@ example:
> >
> >     SEC("lsm/file_mprotect")
> >     int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
> > -            unsigned long reqprot, unsigned long prot, int ret)
> > +            unsigned long reqprot, unsigned long prot)
> >     {
> > -       /* ret is the return value from the previous BPF program
> > -        * or 0 if it's the first hook.
> > -        */
> > -       if (ret != 0)
> > -           return ret;
> > -
>
> This is correct as of today, the return value is checked implicitly by
> the generated trampoline and the next program in the chain is only
> called if this is zero as BPF LSM programs use the modify return
> trampoline:
>
> in invoke_bpf_mod_ret:
>
> /* mod_ret prog stored return value into [rbp - 8]. Emit:
> * if (*(u64 *)(rbp - 8) != 0)
> * goto do_fexit;
> */
> /* cmp QWORD PTR [rbp - 0x8], 0x0 */
> EMIT4(0x48, 0x83, 0x7d, 0xf8); EMIT1(0x00);
>
> Acked-by: KP Singh <kpsingh@...nel.org>