From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from 66-220-144-179.mail-mxout.facebook.com (66-220-144-179.mail-mxout.facebook.com [66.220.144.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D92B419ABD8 for ; Thu, 18 Jun 2026 01:14:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=66.220.144.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781745253; cv=none; b=qorZJEb6tfJGSictNTT85NOjxGr7lOAmWH67HWGfTmepONsZdycRyRwtamDM15lAIxyRtwi7F+35Dyj7b58AcbWZGt5z0X76sZxkgFtjWQOlbrUHJsy0HhlN92XEqKbHV9uceMf4NfdMgvaptrMzQ/CQTOeeT48WchnzftNlH1U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781745253; c=relaxed/simple; bh=s/ulRbLTRRk3nN0pkadw8NppcNvGA6t+O6ONrUPxOeQ=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=K6QT0bGE34m3K9UnjJQQAb2YG++OT8uJNwZlBwUTISNLPYKGtE/BYOvDcY18ZMoH+gzoYzU0l/WvgLosA3A2FRw8OQvtm5G7iesvWHF/s6GvF5jvou2y/pvEBJ3OS4Cng1GAwIL9xVJ7cItvyu+5GjyOEd/Bl7212WU7hHsM27A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev; spf=fail smtp.mailfrom=linux.dev; arc=none smtp.client-ip=66.220.144.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=linux.dev Received: by devvm16039.vll0.facebook.com (Postfix, from userid 128203) id 69C0F176E3B7A0; Wed, 17 Jun 2026 18:13:58 -0700 (PDT) From: Yonghong Song To: Alan Maguire , Arnaldo Carvalho de Melo , dwarves@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , bpf@vger.kernel.org, kernel-team@fb.com Subject: [PATCH dwarves v6 0/5] pahole: Encode true signatures in kernel BTF Date: Wed, 17 Jun 2026 18:13:58 -0700 Message-ID: <20260618011358.632394-1-yonghong.song@linux.dev> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Current vmlinux BTF encoding is based on the source level signatures. But the compiler may do some optimization and changed the signature. If the user tried with source level signature, their initial implementati= on may have wrong results and then the user need to check what is the problem and work around it, e.g. through kprobe since kprobe does not need vmlinux BTF. Majority of changed signatures are due to dead argument elimination. The following is a more complex one. The original source signature: typedef struct { union { void *kernel; void __user *user; }; bool is_kernel : 1; } sockptr_t; typedef sockptr_t bpfptr_t; static int map_create(union bpf_attr *attr, bpfptr_t uattr) { ... } After compiler optimization, the signature becomes: static int map_create(union bpf_attr *attr, bool uattr__is_kernel) { ..= . } In the above, uattr__is_kernel corresponds to 'is_kernel' field in sockpt= r_t. This makes it easier for developers to understand what changed. The new signature needs to properly follow ABI specification based on locations. Otherwise, that signature should be discarded. For example, 0x0242f1f7: DW_TAG_subprogram DW_AT_name ("memblock_find_in_range") DW_AT_calling_convention (DW_CC_nocall) DW_AT_type (0x0242decc "phys_addr_t") ... 0x0242f22e: DW_TAG_formal_parameter DW_AT_location (indexed (0x14a) loclist =3D = 0x005595bc: [0xffffffff87a000f9, 0xffffffff87a00178): DW_OP_= reg5 RDI [0xffffffff87a00178, 0xffffffff87a001be): DW_OP_= reg14 R14 [0xffffffff87a001be, 0xffffffff87a001c7): DW_OP_= entry_value(DW_OP_reg5 RDI), DW_OP_stack_value [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_= reg14 R14) DW_AT_name ("start") DW_AT_type (0x0242decc "phys_addr_t") ... 0x0242f239: DW_TAG_formal_parameter DW_AT_location (indexed (0x14b) loclist =3D = 0x005595e6: [0xffffffff87a000f9, 0xffffffff87a00175): DW_OP_= reg4 RSI [0xffffffff87a00175, 0xffffffff87a001b8): DW_OP_= reg3 RBX [0xffffffff87a001b8, 0xffffffff87a001c7): DW_OP_= entry_value(DW_OP_reg4 RSI), DW_OP_stack_value [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_= reg3 RBX) DW_AT_name ("end") DW_AT_type (0x0242decc "phys_addr_t") ... 0x0242f245: DW_TAG_formal_parameter DW_AT_location (indexed (0x14c) loclist =3D = 0x00559610: [0xffffffff87a001e3, 0xffffffff87a001ef): DW_OP_= breg4 RSI+0) DW_AT_name ("size") DW_AT_type (0x0242decc "phys_addr_t") ... 0x0242f250: DW_TAG_formal_parameter DW_AT_const_value (4096) DW_AT_name ("align") DW_AT_type (0x0242decc "phys_addr_t") ... The third argument should correspond to RDX for x86_64. But the location = suggests that the parameter value is stored in the address with 'RSI + 0'. It is not cl= ear whether the parameter value is stored in RDX or not. So we have to discard this f= unciton in vmlinux BTF to avoid incorrect true signatures. For llvm, any function having DW_AT_calling_convention (DW_CC_nocall) in dwarf DW_TAG_subprogram will indicate that this function has signature= changed. I did experiment with latest bpf-next. For x86_64, there are 69103 kernel= functions and 875 kernel functions having signature changed. A series of patches ar= e intended to ensure true signatures are properly represented. Eventually, only 20 f= unctions cannot have true signatures due to locations. For arm64, there are 863 kernel functions having signature changed, and 108 functions cannot have true signatures due to locations. I checked tho= se functions and look like llvm arm64 backend more relaxed to compute parame= ter values. For full testing, I enabled true signature support in kernel scripts/Make= file.btf like below: -pahole-flags-$(call test-ge, $(pahole-ver), 131) +=3D --btf_features=3D= attributes +pahole-flags-$(call test-ge, $(pahole-ver), 131) +=3D --btf_features=3D= attributes --btf_features=3D+true_signature See individual patches for details. Changelog: v5 -> v6: - v5: https://lore.kernel.org/bpf/20260523165712.1225231-1-yonghong.s= ong@linux.dev/ - The previous change relies on parameter__new() to collect and analy= ze each parameter to decide true signatures. The new one separates collecti= ng and analyzing phase from Alan. This two-phase makes logic easy to under= stand. - In btf_encoder.c, remove usage of skip_idx to simplify the code. v4 -> v5: - v4: https://lore.kernel.org/bpf/20260326013144.2901265-1-yonghong.s= ong@linux.dev/ - Check info.signature_changed only under clang. - Fix an uninitialized varable issue (var reg_dix) for gcc. v3 -> v4: - v3: https://lore.kernel.org/bpf/20260320190917.1970524-1-yonghong.s= ong@linux.dev/ - Add simple prescan of parameter registers in order to get true sign= atures for those functions where optimization could happen but compiler di= dn't do it. - Do not create a new name (e.g. "uattr__is_kernel") with malloc at p= arameter_reg() stage. Instead remember both "uattr" and "is_kernel" and later gene= rate the name "uattr_is_kernel" in btf encoder. - Add comments to explain how to handle parameters which may take two= registers. - Fix some test failures on aarch64. v2 -> v3: - v2: https://lore.kernel.org/bpf/20260309153215.1917033-1-yonghong.s= ong@linux.dev/ - Change tests by using newly added test_lib.sh. - Simplify to get bool variable producer_clang. - Try to avoid producer_clang appearance in dwarf_loader.c in order t= o avoid clear separation between clang and gcc. v1 -> v2: - v1: https://lore.kernel.org/bpf/20260305225455.1151066-1-yonghong.s= ong@linux.dev/ - Added producer_clang guarding in btf_encoder. Otherwise, gcc kernel= build will crash pahole. - Fix an early return in parameter__reg() which didn't do pthread_mut= ex_unlock() which caused the deadlock for arm64. - Add a few more places to guard with producer_clang and conf->true_s= ignature to maintain the previous behavior if not clang or conf->true_signat= ure is false. Yonghong Song (5): dwarf_loader: Detect aggregate ABI register usage and signature changes dwarf_loader: Collect per-parameter information dwarf_loader: Analyze per-parameter information for true signatures btf_encoder: Emit true function signatures tests: add BTF true_signature encoding tests btf_encoder.c | 24 +- dwarf_loader.c | 548 ++++++++++++++++++++++++---- dwarves.h | 14 + tests/clang_parm_aggregate.sh | 85 +++++ tests/clang_parm_memory.sh | 77 ++++ tests/clang_parm_optimized.sh | 63 ++++ tests/clang_parm_optimized_stack.sh | 63 ++++ 7 files changed, 812 insertions(+), 62 deletions(-) create mode 100755 tests/clang_parm_aggregate.sh create mode 100755 tests/clang_parm_memory.sh create mode 100755 tests/clang_parm_optimized.sh create mode 100755 tests/clang_parm_optimized_stack.sh --=20 2.53.0-Meta