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Fri, 13 Nov 2026 21:58:42 -0800 (PST) Received: from localhost ([153.61.198.243]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2ee0f862744sm17702925ad.44.2026.11.13.21.58.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 13 Nov 2026 21:58:41 -0800 (PST) Content-Type: text/plain; charset=UTF-8 Date: Sat, 14 Nov 2026 05:58:41 +0000 Message-Id: From: "Alexei Starovoitov" To: "Hungyu Lin" , , Cc: , , , , , , , , , , Subject: Re: [PATCH bpf-next] libbpf: Fix sign extension of unaligned enums in btf_dump_get_enum_value() In-Reply-To: <20261113223449.97604-1-dennylin0707@gmail.com> References: <20261113223449.97604-1-dennylin0707@gmail.com> Content-Transfer-Encoding: 8bit X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Fri, Nov 13, 2026 at 02:34 PM Hungyu Lin wrote: > btf_dump_get_enum_value() reads unaligned enum values through > btf_dump_get_bitfield_value() with a zero bit size. That path > zero-extends the value instead of using the signed loads that are used Not quite. bit_sz == 0 makes right_shift_bits == 64 in *value = (num << left_shift_bits) >> right_shift_bits; That's undefined behavior for __u64 on both endians. Pls say it in the commit log. > + if (t->size <= sizeof(buf) && !ptr_is_aligned(d->btf, id, data)) { > + if (data + t->size > d->typed_dump->data_end) > + return -E2BIG; This breaks the dump of struct fs_context when emit_zeroes is not set. 'phase' is an enum bitfield of 8 bits, 3 bytes before the end of the struct. btf_dump_type_data_check_zero() ignores bit_sz of an enum: btf_dump_struct_data() btf_dump_dump_type_data(data = end - 3, bit_sz = 8) btf_dump_type_data_check_overflow() 1 byte, ok btf_dump_type_data_check_zero() btf_dump_get_enum_value() t->size == 4, unaligned end - 3 + 4 > data_end -E2BIG Today it reads 1 byte past data_end and the dump succeeds. The over-read is an old bug of enum bitfields. It needs its own fix. The fs_context test in test_btf_dump_struct_data() sets emit_zeroes, so it doesn't notice. Drop the check. btf_dump_int_data() copies without it. The check against data_end belongs in one place for all types. The first case below fails on bpf-next today. The second case fails with this patch. I didn't build it and didn't run it: diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c index b5c90f7..d6fd993 100644 --- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c +++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c @@ -945,7 +945,7 @@ static void test_btf_dump_enum_data(struct btf *btf, struct btf_dump *d, char *str) { __u32 high = 0x80000000; - __s32 neg = -1; + __s32 neg[2] = { -1, -1 }; /* enum where enum value does (and does not) exist */ TEST_BTF_DUMP_DATA_C(btf, d, "enum", str, enum bpf_cmd, BTF_F_COMPACT, @@ -994,6 +994,8 @@ static void test_btf_dump_enum_data(struct btf *btf, struct btf_dump *d, "(enum test_enum_u32)TEST_ENUM_HIGH"), "dump unsigned enum"); ASSERT_OK(btf_dump_data(btf, d, "test_enum_s32", NULL, 0, &neg, 4, str, "(enum test_enum_s32)TEST_ENUM_NEG"), "dump signed enum"); + ASSERT_OK(btf_dump_data(btf, d, "test_enum_s32", NULL, 0, (void *)neg + 1, 4, str, + "(enum test_enum_s32)TEST_ENUM_NEG"), "dump unaligned enum"); btf_dump__free(d); } @@ -1182,6 +1184,12 @@ static void test_btf_dump_struct_data(struct btf *btf, struct btf_dump *d, type_sz = btf__resolve_size(btf, type_id); str[0] = '\0'; + /* 'phase' is an enum bitfield 3 bytes before the end */ + ret = btf_dump__dump_type_data(d, type_id, zero_data, type_sz, &opts); + ASSERT_EQ(ret, type_sz, + "unexpected return value dumping zeroed fs_context"); + + str[0] = '\0'; opts.emit_zeroes = true; ret = btf_dump__dump_type_data(d, type_id, zero_data, type_sz, &opts); ASSERT_EQ(ret, type_sz, pw-bot: cr