Subject: Re: [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs

From: Jessica Yu <[email-masked]>
Date: Wed, 11 Nov 2020 11:13:17 +0100

+++ 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
>