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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-49c60f8d615sm16954821f8f.56.2026.11.15.06.37.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 15 Nov 2026 06:37:35 -0800 (PST) Date: Sun, 15 Nov 2026 14:37:34 +0000 From: David Laight To: "Alexei Starovoitov" Cc: "Hungyu Lin" , , , , , , , , , , , , , Subject: Re: [PATCH bpf-next] libbpf: Fix sign extension of unaligned enums in btf_dump_get_enum_value() Message-ID: <20261115143734.4addeb28@pumpkin> In-Reply-To: References: <20261113223449.97604-1-dennylin0707@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) 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 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sat, 14 Nov 2026 05:58:41 +0000 "Alexei Starovoitov" wrote: > 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. Isn't a size of 0 just wrong? The code is likely also wrong for a size of 64 - which is valid. I also remember there being a helper somewhere to sign extend a 'narrow' integer. David > > > + 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 >