linux-kernel - Re: [PATCH bpf-next v7 04/11] bpf: support fsession for bpf_session_is

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Message-Id: <7b965f06828236a16f0146550e01a581b1c202c427584a894d5a70a42ff06435@mail.kernel.org>
Date: Wed,  7 Jan 2026 07:11:12 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: menglong8.dong@...il.com,ast@...nel.org,andrii@...nel.org
Cc: daniel@...earbox.net,martin.lau@...ux.dev,eddyz87@...il.com,song@...nel.org,yonghong.song@...ux.dev,john.fastabend@...il.com,kpsingh@...nel.org,sdf@...ichev.me,haoluo@...gle.com,jolsa@...nel.org,davem@...emloft.net,dsahern@...nel.org,tglx@...utronix.de,mingo@...hat.com,jiang.biao@...ux.dev,bp@...en8.de,dave.hansen@...ux.intel.com,x86@...nel.org,hpa@...or.com,bpf@...r.kernel.org,netdev@...r.kernel.org,linux-kernel@...r.kernel.org,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v7 04/11] bpf: support fsession for bpf_session_is_return

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 41228b0add52..95248b0d28ab 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1229,6 +1229,9 @@ enum {
>  #endif
>  };
>
> +#define BPF_TRAMP_M_NR_ARGS	0
> +#define BPF_TRAMP_M_IS_RETURN	8

[ ... ]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index bfff3f84fd91..d3709edd0e51 100644

[ ... ]

> @@ -22558,6 +22564,13 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
>  		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
>  		*cnt = 1;
> +	} else if (desc->func_id == special_kfunc_list[KF_bpf_session_is_return] &&
> +		   env->prog->expected_attach_type == BPF_TRACE_FSESSION) {
> +		/* Load nr_args from ctx - 8 */
> +		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +		insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_M_IS_RETURN);
> +		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
> +		*cnt = 3;
                ^^^^

This inlined code reads the BPF_TRAMP_M_IS_RETURN bit (bit 8) from ctx[-1]
to determine if the program is running at function exit. However, looking
at arch/x86/net/bpf_jit_comp.c, the trampoline only stores nr_regs to
ctx[-8] at this point in the series:

    emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_regs);
    emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -nregs_off);

The BPF_TRAMP_M_IS_RETURN flag is not being set by the trampoline code.
Is this intentional? The arch code that sets this bit appears to be added
in a later commit in the series (93d4641b9513 "bpf,x86: add fsession
support for x86_64").

At this commit, bpf_session_is_return() for fsession programs will always
return false, even during fexit execution, because bit 8 is never set.

> +	}
>
>  	if (env->insn_aux_data[insn_idx].arg_prog) {

[ ... ]

> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 736b32cf2195..9d3bf3bbe8f6 100644

[ ... ]

> @@ -3314,6 +3314,12 @@ static u64 bpf_uprobe_multi_entry_ip(struct bpf_run_ctx *ctx)
>  }
>  #endif /* CONFIG_UPROBES */
>
> +bool bpf_fsession_is_return(void *ctx)
> +{
> +	/* This helper call is inlined by verifier. */
> +	return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
> +}

The same issue exists here in bpf_fsession_is_return() - it reads bit 8
which is not set by the trampoline at this point in the series. The
comment indicates this is inlined by the verifier, but both code paths
have the same problem.

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20773233136