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Sat, 17 Oct 2026 04:59:02 -0700 (PDT) Received: from krava ([2a02:8308:a00c:e200::25dc]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a1d7659805sm76775585e9.7.2026.10.17.04.58.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 Oct 2026 04:59:01 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sat, 17 Oct 2026 13:58:58 +0200 To: Woojin Ji Cc: bpf@vger.kernel.org Subject: Re: [PATCH bpf] bpf: Zero-extend the index in inlined bpf_get_func_arg() Message-ID: References: <179221432098.1890560.17043109037413683447.get-func-arg-index@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <179221432098.1890560.17043109037413683447.get-func-arg-index@gmail.com> On Sat, Oct 17, 2026 at 02:42:24PM +0900, Woojin Ji wrote: > bpf_get_func_arg() takes a u32 index, but the inlined expansion compares > only the low 32 bits of R2 with the argument count and then shifts the > full 64-bit register to compute the context address. R2 is ARG_ANYTHING, > so nothing guarantees that its upper bits are zero: an index of > 1ULL << 32 passes the bounds check as index zero while the load is > performed at ctx + (1ULL << 35). > > Zero-extend R2 to the u32 index the helper takes, which is what the C > implementation does; that path is still reachable because LSM programs > with BPF_LSM_MAC are not inlined. Widening the comparison instead would > reject indices whose low 32 bits are valid. Since the check is 32-bit, > this cannot change which programs are accepted, only the address. > > Extend the existing fexit and tp_btf tests with a 64-bit helper > prototype so the upper index bits reach the expansion. > > Assisted-by: LLM > Fixes: f92c1e183604 ("bpf: Add get_func_[arg|ret|arg_cnt] helpers") > Signed-off-by: Woojin Ji > --- > Validation: > - Built the modified objects and both affected test programs. > - Disposable x86_64 QEMU guest, clang-22 kernel with BTF, tracing and > bpf_testmod: both tests pass (2/0 PASSED, 0 SKIPPED, 0 FAILED) and > the guest dmesg is clean. > - Same tests on unpatched ff47652a4b66: the wide index faults in the > inline sequence and the guest panics. > > kernel/bpf/fixups.c | 5 +++ > .../selftests/bpf/progs/get_func_args_test.c | 36 ++++++++++++++++++++++ > 2 files changed, 41 insertions(+) nice, please split kernel and test into separate patches Acked-by: Jiri Olsa thanks, jirka > > diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c > index d6f83521fc78e6c6a82d1969662f1eb52f64f4c3..005ed9279ebab60a8a774c4e6937232ab4f38962 100644 > --- a/kernel/bpf/fixups.c > +++ b/kernel/bpf/fixups.c > @@ -2255,6 +2255,11 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env) > insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF); > cnt = 2; > } > + /* > + * The helper takes a u32 index; zero-extend it before > + * the bounds check and the 64-bit address math below. > + */ > + insn_buf[cnt++] = BPF_MOV32_REG(BPF_REG_2, BPF_REG_2); > insn_buf[cnt++] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6); > insn_buf[cnt++] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3); > insn_buf[cnt++] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1); > diff --git a/tools/testing/selftests/bpf/progs/get_func_args_test.c b/tools/testing/selftests/bpf/progs/get_func_args_test.c > index 1bf47f64d0967a3543739841201b58b14b4036a7..9b0beb58d88d5bcf1a36a0598695fd02a2cebbcf 100644 > --- a/tools/testing/selftests/bpf/progs/get_func_args_test.c > +++ b/tools/testing/selftests/bpf/progs/get_func_args_test.c > @@ -6,6 +6,40 @@ > > char _license[] SEC("license") = "GPL"; > > +/* Keep the upper index bits: the inlined expansion must truncate them. */ > +static long (*bpf_get_func_arg_wide)(void *ctx, __u64 n, __u64 *value) = > + (void *)BPF_FUNC_get_func_arg; > + > +/* Both traced functions are called with arguments 2 and 3. */ > +static __always_inline int check_wide_index(void *ctx) > +{ > + __u64 a = 0, z = 0xdeadbeefdeadbeefULL; > + __s64 err; > + int ok = 1; > + > + /* The index is truncated to u32, so these select arguments 0 and 1. */ > + err = bpf_get_func_arg_wide(ctx, 1ULL << 32, &a); > + ok &= err == 0 && ((int)a == 2); > + err = bpf_get_func_arg_wide(ctx, (1ULL << 32) | 1, &a); > + ok &= err == 0 && a == 3; > + err = bpf_get_func_arg_wide(ctx, 1ULL << 59, &a); > + ok &= err == 0 && ((int)a == 2); > + err = bpf_get_func_arg_wide(ctx, (1ULL << 59) | 1, &a); > + ok &= err == 0 && a == 3; > + > + /* Out of range: -EINVAL and no store into the destination. */ > + err = bpf_get_func_arg_wide(ctx, (1ULL << 32) | 2, &z); > + ok &= err == -EINVAL && z == 0xdeadbeefdeadbeefULL; > + err = bpf_get_func_arg_wide(ctx, (1ULL << 32) | 0xffffffffULL, &z); > + ok &= err == -EINVAL && z == 0xdeadbeefdeadbeefULL; > + err = bpf_get_func_arg_wide(ctx, (1ULL << 59) | 2, &z); > + ok &= err == -EINVAL && z == 0xdeadbeefdeadbeefULL; > + err = bpf_get_func_arg_wide(ctx, (1ULL << 59) | 0xffffffffULL, &z); > + ok &= err == -EINVAL && z == 0xdeadbeefdeadbeefULL; > + > + return ok; > +} > + > __u64 test1_result = 0; > SEC("fentry/bpf_fentry_test1") > int BPF_PROG(test1) > @@ -63,6 +97,7 @@ int BPF_PROG(test2) > /* return value */ > err = bpf_get_func_ret(ctx, &ret); > test2_result &= err == 0 && ret == 5; > + test2_result &= check_wide_index(ctx); > return 0; > } > > @@ -162,6 +197,7 @@ int BPF_PROG(tp_test2) > /* not valid argument */ > err = bpf_get_func_arg(ctx, 2, &z); > test6_result &= err == -EINVAL; > + test6_result &= check_wide_index(ctx); > > return 0; > } > > base-commit: ff47652a4b66c067c765a7ad464d930b5a9367cc > -- > 2.47.3 >