module: Factor out elf_validity_cache_index_mod

Centralize .gnu.linkonce.this_module detection and property validation.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

kernel/module/main.c

@@ -1919,6 +1919,68 @@ static int elf_validity_cache_index_info(struct load_info *info)
 	return 0;
 }
 
+/**
+ * elf_validity_cache_index_mod() - Validates and caches this_module section
+ * @info: Load info to cache this_module on.
+ *        Must have &load_info->sechdrs and &load_info->secstrings populated
+ *
+ * The ".gnu.linkonce.this_module" ELF section is special. It is what modpost
+ * uses to refer to __this_module and let's use rely on THIS_MODULE to point
+ * to &__this_module properly. The kernel's modpost declares it on each
+ * modules's *.mod.c file. If the struct module of the kernel changes a full
+ * kernel rebuild is required.
+ *
+ * We have a few expectations for this special section, this function
+ * validates all this for us:
+ *
+ * * The section has contents
+ * * The section is unique
+ * * We expect the kernel to always have to allocate it: SHF_ALLOC
+ * * The section size must match the kernel's run time's struct module
+ *   size
+ *
+ * If all checks pass, the index will be cached in &load_info->index.mod
+ *
+ * Return: %0 on validation success, %-ENOEXEC on failure
+ */
+static int elf_validity_cache_index_mod(struct load_info *info)
+{
+	Elf_Shdr *shdr;
+	int mod_idx;
+
+	mod_idx = find_any_unique_sec(info, ".gnu.linkonce.this_module");
+	if (mod_idx <= 0) {
+		pr_err("module %s: Exactly one .gnu.linkonce.this_module section must exist.\n",
+		       info->name ?: "(missing .modinfo section or name field)");
+		return -ENOEXEC;
+	}
+
+	shdr = &info->sechdrs[mod_idx];
+
+	if (shdr->sh_type == SHT_NOBITS) {
+		pr_err("module %s: .gnu.linkonce.this_module section must have a size set\n",
+		       info->name ?: "(missing .modinfo section or name field)");
+		return -ENOEXEC;
+	}
+
+	if (!(shdr->sh_flags & SHF_ALLOC)) {
+		pr_err("module %s: .gnu.linkonce.this_module must occupy memory during process execution\n",
+		       info->name ?: "(missing .modinfo section or name field)");
+		return -ENOEXEC;
+	}
+
+	if (shdr->sh_size != sizeof(struct module)) {
+		pr_err("module %s: .gnu.linkonce.this_module section size must match the kernel's built struct module size at run time\n",
+		       info->name ?: "(missing .modinfo section or name field)");
+		return -ENOEXEC;
+	}
+
+	info->index.mod = mod_idx;
+
+	return 0;
+}
+
+
 /*
  * Check userspace passed ELF module against our expectations, and cache
  * useful variables for further processing as we go.
@@ -1944,7 +2006,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	unsigned int i;
 	Elf_Shdr *shdr;
 	int err;
-	unsigned int num_mod_secs = 0, mod_idx;
 	unsigned int num_sym_secs = 0, sym_idx;
 
 	err = elf_validity_cache_sechdrs(info);
@@ -1954,16 +2015,15 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	if (err < 0)
 		return err;
 	err = elf_validity_cache_index_info(info);
+	if (err < 0)
+		return err;
+	err = elf_validity_cache_index_mod(info);
 	if (err < 0)
 		return err;
 
 	for (i = 1; i < info->hdr->e_shnum; i++) {
 		shdr = &info->sechdrs[i];
-		switch (shdr->sh_type) {
+		if (shdr->sh_type == SHT_SYMTAB) {
-		case SHT_NULL:
-		case SHT_NOBITS:
-			continue;
-		case SHT_SYMTAB:
 			if (shdr->sh_link == SHN_UNDEF
 			    || shdr->sh_link >= info->hdr->e_shnum) {
 				pr_err("Invalid ELF sh_link!=SHN_UNDEF(%d) or (sh_link(%d) >= hdr->e_shnum(%d)\n",
@@ -1973,14 +2033,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 			}
 			num_sym_secs++;
 			sym_idx = i;
-			fallthrough;
-		default:
-			if (strcmp(info->secstrings + shdr->sh_name,
-				   ".gnu.linkonce.this_module") == 0) {
-				num_mod_secs++;
-				mod_idx = i;
-			}
-			break;
 		}
 	}
 
@@ -1996,55 +2048,8 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	info->index.str = shdr->sh_link;
 	info->strtab = (char *)info->hdr + info->sechdrs[info->index.str].sh_offset;
 
-	/*
-	 * The ".gnu.linkonce.this_module" ELF section is special. It is
-	 * what modpost uses to refer to __this_module and let's use rely
-	 * on THIS_MODULE to point to &__this_module properly. The kernel's
-	 * modpost declares it on each modules's *.mod.c file. If the struct
-	 * module of the kernel changes a full kernel rebuild is required.
-	 *
-	 * We have a few expectaions for this special section, the following
-	 * code validates all this for us:
-	 *
-	 *   o Only one section must exist
-	 *   o We expect the kernel to always have to allocate it: SHF_ALLOC
-	 *   o The section size must match the kernel's run time's struct module
-	 *     size
-	 */
-	if (num_mod_secs != 1) {
-		pr_err("module %s: Only one .gnu.linkonce.this_module section must exist.\n",
-		       info->name ?: "(missing .modinfo section or name field)");
-		goto no_exec;
-	}
-
-	shdr = &info->sechdrs[mod_idx];
-
-	/*
-	 * This is already implied on the switch above, however let's be
-	 * pedantic about it.
-	 */
-	if (shdr->sh_type == SHT_NOBITS) {
-		pr_err("module %s: .gnu.linkonce.this_module section must have a size set\n",
-		       info->name ?: "(missing .modinfo section or name field)");
-		goto no_exec;
-	}
-
-	if (!(shdr->sh_flags & SHF_ALLOC)) {
-		pr_err("module %s: .gnu.linkonce.this_module must occupy memory during process execution\n",
-		       info->name ?: "(missing .modinfo section or name field)");
-		goto no_exec;
-	}
-
-	if (shdr->sh_size != sizeof(struct module)) {
-		pr_err("module %s: .gnu.linkonce.this_module section size must match the kernel's built struct module size at run time\n",
-		       info->name ?: "(missing .modinfo section or name field)");
-		goto no_exec;
-	}
-
-	info->index.mod = mod_idx;
-
 	/* This is temporary: point mod into copy of data. */
-	info->mod = (void *)info->hdr + shdr->sh_offset;
+	info->mod = (void *)info->hdr + info->sechdrs[info->index.mod].sh_offset;
 
 	/*
 	 * If we didn't load the .modinfo 'name' field earlier, fall back to