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[34.141.185.49]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b7cfa570174sm371812666b.55.2025.12.11.13.39.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Dec 2025 13:39:52 -0800 (PST) Date: Thu, 11 Dec 2025 21:39:49 +0000 From: Matt Bobrowski To: Andrii Nakryiko , Eduard Zingerman Cc: =?utf-8?B?5qKF5byA5b2m?= , Daniel Borkmann , bpf , Martin KaFai Lau , hust-os-kernel-patches@googlegroups.com, Yinhao Hu , dzm91@hust.edu.cn, KP Singh , Alexei Starovoitov Subject: Re: bpf: mmap_file LSM hook allows NULL pointer dereference Message-ID: References: <5e460d3c.4c3e9.19adde547d8.Coremail.kaiyanm@hust.edu.cn> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Dec 10, 2025 at 10:02:16AM +0000, Matt Bobrowski wrote: > On Wed, Dec 03, 2025 at 10:23:43AM -0800, Alexei Starovoitov wrote: > > On Wed, Dec 3, 2025 at 12:47 AM Matt Bobrowski wrote: > > > > > > > We can play tricks with __weak. Like: > > > > > > > > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c > > > > index 7cb6e8d4282c..60d269a85bf1 100644 > > > > --- a/kernel/bpf/bpf_lsm.c > > > > +++ b/kernel/bpf/bpf_lsm.c > > > > @@ -21,7 +21,7 @@ > > > > * function where a BPF program can be attached. > > > > */ > > > > #define LSM_HOOK(RET, DEFAULT, NAME, ...) \ > > > > -noinline RET bpf_lsm_##NAME(__VA_ARGS__) \ > > > > +__weak noinline RET bpf_lsm_##NAME(__VA_ARGS__) \ > > > > > > > > diff kernel/bpf/bpf_lsm_proto.c > > > > > > > > +int bpf_lsm_mmap_file(struct file *file__nullable, unsigned long reqprot, > > > > + unsigned long prot, unsigned long flags) > > > > +{ > > > > + return 0; > > > > +} > > > > > > > > and above one with __nullable will be in vmlinux BTF. > > > > > > > > afaik __weak functions are not removed by linker when in non-LTO, > > > > but it's still better than > > > > +#define bpf_lsm_mmap_file bpf_lsm_mmap_file__original > > > > No need to change bpf_lsm.h either. > > > > > > Annotating with a weak attribute would be quite nice, but the compiler > > > will complain about the redefinition of the symbol > > > bpf_lsm_mmap_file. To avoid this, we'd still need to rely on the > > > rename and ignore dance by using the aforementioned define, which at > > > that point would still result in both symbols being exposed in both > > > BTF and the .text section. > > > > Not quite. You missed this part in the above: > > > > > > diff kernel/bpf/bpf_lsm_proto.c > > > > it's a different file. > > Yes, yes, this will work. However, as discussed, it's fundamentally > reliant on a small "hack" which I've implemented within > kernel/bpf/Makefile here [0] to workaround current pahole > deduplication logic. > > Andrii and Eduard, > > I’d like your input on a pahole BTF generation issue which I've > recently come across. In the series I just sent [0], I had to > implement a workaround to force pahole to process bpf_lsm_proto.o > before bpf_lsm.o. > > This was necessary to ensure pahole generates BTF for the strong > definition of bpf_lsm_mmap_file() (in bpf_lsm_proto.c) rather than the > weak definition (in bpf_lsm.c). Without this forced ordering, pahole > processed the weak definition first, resulting in a state array like > this: > > ``` > btf_encoder.func_states.array[N] = bpf_lsm_mmap_file (weak > definition from bpf_lsm.o) > > btf_encoder.func_states.array[N+1] = bpf_lsm_mmap_file (strong > definition from bpf_lsm_proto.o) > ``` > > Because the deduplication logic in btf_encoder__add_saved_funcs() > folds duplicates (those determined by saved_functions_combine()) into > the first occurrence, the resulting BTF was derived from the weak > definition. This is incorrect, as the strong definition is the one > actually linked into the final vmlinux image. > > An obvious fix that immediately came to mind here was to essentially > teach pahole about strong function prototype definitions, and prefer > to emit BTF for those instead of any weak defined counterparts? Thinking about this a little more. Perhaps whilst in btf_encoder__add_saved_funcs() we should only emit BTF for any duplicated function within a CU which happen to match the corresponding entry within the backing ELF symtab? We can do this by checking whether the virtual address stored within DW_AT_low_pc matches that of what's stored in the st_value field for the corresponding ELF symtab entry? For example, for bpf_lsm_mmap_file we have: Output from reading the vmlinux symbol table: ``` $ readelf -s | grep bpf_lsm_mmap_file 165360: ffffffff8152f9b0 16 FUNC GLOBAL DEFAULT 1 bpf_lsm_mmap_file ``` Output from reading the vmlinux DWARF debugging information: ``` <2a40982> DW_AT_name : (indirect string, offset: 0x1352ea): bpf_lsm_mmap_file <2a40986> DW_AT_decl_file : 4 <2a40987> DW_AT_decl_line : 199 <2a40988> DW_AT_decl_column : 1 <2a40989> DW_AT_prototyped : 1 <2a40989> DW_AT_type : <0x2a1b010> <2a4098d> DW_AT_low_pc : 0xffffffff8152e260 <2a40995> DW_AT_high_pc : 0x10 <2a4099d> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa) <2a4099f> DW_AT_call_all_calls: 1 <2a4099f> DW_AT_sibling : <0x2a409d8> <2><2a409a3>: Abbrev Number: 10 (DW_TAG_formal_parameter) <2a409a4> DW_AT_name : (indirect string, offset: 0x3623df): file <2a409a8> DW_AT_decl_file : 4 <2a409a9> DW_AT_decl_line : 199 <2a409aa> DW_AT_decl_column : 1 <2a409aa> DW_AT_type : <0x2a234ef> <2a409ae> DW_AT_location : 1 byte block: 55 (DW_OP_reg5 (rdi)) <2><2a409b0>: Abbrev Number: 10 (DW_TAG_formal_parameter) <2a409b1> DW_AT_name : (indirect string, offset: 0x23a09d): reqprot <2a409b5> DW_AT_decl_file : 4 -- <2a60e0a> DW_AT_name : (indirect string, offset: 0x1352ea): bpf_lsm_mmap_file <2a60e0e> DW_AT_decl_file : 1 <2a60e0f> DW_AT_decl_line : 15 <2a60e10> DW_AT_decl_column : 5 <2a60e11> DW_AT_prototyped : 1 <2a60e11> DW_AT_type : <0x2a42713> <2a60e15> DW_AT_low_pc : 0xffffffff8152f9b0 <2a60e1d> DW_AT_high_pc : 0x10 <2a60e25> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa) <2a60e27> DW_AT_call_all_calls: 1 <2><2a60e27>: Abbrev Number: 82 (DW_TAG_formal_parameter) <2a60e28> DW_AT_name : (indirect string, offset: 0x135ede): file__nullable <2a60e2c> DW_AT_decl_file : 1 <2a60e2c> DW_AT_decl_line : 15 <2a60e2d> DW_AT_decl_column : 36 <2a60e2e> DW_AT_type : <0x2a49f59> <2a60e32> DW_AT_location : 1 byte block: 55 (DW_OP_reg5 (rdi)) ``` > [0] https://lore.kernel.org/bpf/20251210090701.2753545-1-mattbobrowski@google.com/T/#me14d534fb559a349c46e094f18c63d477644d511