Re: [PATCH bpf-next v9 05/11] bpf: support fsession for bpf_session_cookie
From: bot+bpf-ci@kernel.org
Date: 2026-01-10 14:42:39
Also in:
bpf, lkml
quoted hunk ↗ jump to hunk
diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 2640ec2157e1..a416050e0dd2 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h@@ -1231,6 +1231,7 @@ enum { #define BPF_TRAMP_M_NR_ARGS 0 #define BPF_TRAMP_M_IS_RETURN 8 +#define BPF_TRAMP_M_COOKIE 9
[ ... ]
quoted hunk ↗ jump to hunk
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 1b0292a03186..b91fd8af2393 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c
[ ... ]
quoted hunk ↗ jump to hunk
@@ -22571,6 +22575,22 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, 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; + } else if (desc->func_id == special_kfunc_list[KF_bpf_session_cookie] && + env->prog->expected_attach_type == BPF_TRACE_FSESSION) { + /* inline bpf_session_cookie() for fsession: + * __u64 *bpf_session_cookie(void *ctx) + * { + * u64 off = (((u64 *)ctx)[-1] >> BPF_TRAMP_M_COOKIE) & 0xFF; + * return &((u64 *)ctx)[-off]; + * } + */ + 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_COOKIE); + insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF); + insn_buf[3] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3); + insn_buf[4] = BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1); + insn_buf[5] = BPF_ALU64_IMM(BPF_NEG, BPF_REG_0, 0);
^^^^^^^^
This inline code reads the cookie offset from (ctx[-1] >> 9) & 0xFF, but
does the x86 trampoline actually store the cookie offset at bit position 9?
Looking at arch/x86/net/bpf_jit_comp.c in __arch_prepare_bpf_trampoline(),
the trampoline stores nr_regs to the nregs_off location:
emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_regs);
emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -nregs_off);
This sets ctx[-1] to a small value like 0x0000000000000006, with the high
bits all zero. When the inline code executes:
off = (ctx[-1] >> 9) & 0xFF --> off = 0
The function then returns &ctx[-0] = ctx itself, which points to the
function arguments instead of the session cookie.
Is there a missing dependency on a later commit in the series? Commit
983968c39241 ("bpf,x86: add fsession support for x86_64") appears to add
the trampoline code that stores the cookie offset, but it comes after this
commit in the patch series.
+ *cnt = 6; + }
--- 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/20879693280