Messages in this thread Patch in this message | | | Date | Wed, 11 Nov 2020 11:13:17 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs |
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+++ Andrii Nakryiko [09/11/20 17:19 -0800]: [snipped] >diff --git a/kernel/module.c b/kernel/module.c >index a4fa44a652a7..f2996b02ab2e 100644 >--- a/kernel/module.c >+++ b/kernel/module.c >@@ -380,6 +380,35 @@ static void *section_objs(const struct load_info *info, > return (void *)info->sechdrs[sec].sh_addr; > } > >+/* Find a module section: 0 means not found. Ignores SHF_ALLOC flag. */ >+static unsigned int find_any_sec(const struct load_info *info, const char *name) >+{ >+ unsigned int i; >+ >+ for (i = 1; i < info->hdr->e_shnum; i++) { >+ Elf_Shdr *shdr = &info->sechdrs[i]; >+ if (strcmp(info->secstrings + shdr->sh_name, name) == 0) >+ return i; >+ } >+ return 0; >+} >+ >+/* >+ * Find a module section, or NULL. Fill in number of "objects" in section. >+ * Ignores SHF_ALLOC flag. >+ */ >+static __maybe_unused void *any_section_objs(const struct load_info *info, >+ const char *name, >+ size_t object_size, >+ unsigned int *num) >+{ >+ unsigned int sec = find_any_sec(info, name); >+ >+ /* Section 0 has sh_addr 0 and sh_size 0. */ >+ *num = info->sechdrs[sec].sh_size / object_size; >+ return (void *)info->sechdrs[sec].sh_addr; >+} >+
Hm, I see this patchset has already been applied to bpf-next, but I guess that doesn't preclude any follow-up patches :-)
I am not a huge fan of the code duplication here, and also the fact that they're only called in one place. any_section_objs() and find_any_sec() are pretty much identical to section_objs() and find_sec(), other than the fact the former drops the SHF_ALLOC check.
Moreover, since it appears that the ".BTF" section is not marked SHF_ALLOC, I think this will leave mod->btf_data as a dangling pointer after the module is done loading and the module's load_info has been deallocated, since SHF_ALLOC sections are not allocated nor copied to the module's final location in memory.
Why not simply mark the ".BTF" section in the module SHF_ALLOC? We already do some sh_flags rewriting in rewrite_section_headers(). Then the module loader knows to keep the section in memory and you can use section_objs(). And since the .BTF section stays in module memory, that might save you the memcpy() to btf->data in btf_parse_module() (unless that is still needed for some reason).
Thanks,
Jessica > /* Provided by the linker */ > extern const struct kernel_symbol __start___ksymtab[]; > extern const struct kernel_symbol __stop___ksymtab[]; >@@ -3250,6 +3279,9 @@ static int find_module_sections(struct module *mod, struct load_info *info) > sizeof(*mod->bpf_raw_events), > &mod->num_bpf_raw_events); > #endif >+#ifdef CONFIG_DEBUG_INFO_BTF_MODULES >+ mod->btf_data = any_section_objs(info, ".BTF", 1, &mod->btf_data_size); >+#endif > #ifdef CONFIG_JUMP_LABEL > mod->jump_entries = section_objs(info, "__jump_table", > sizeof(*mod->jump_entries), >-- >2.24.1 >
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