arm64: module: rework special section handling

When we load a module, we have to perform some special work for a couple
of named sections. To do this, we iterate over all of the module's
sections, and perform work for each section we recognize.

To make it easier to handle the unexpected absence of a section, and to
make the section-specific logic easer to read, let's factor the section
search into a helper. Similar is already done in the core module loader,
and other architectures (and ideally we'd unify these in future).

If we expect a module to have an ftrace trampoline section, but it
doesn't have one, we'll now reject loading the module. When
ARM64_MODULE_PLTS is selected, any correctly built module should have
one (and this is assumed by arm64's ftrace PLT code) and the absence of
such a section implies something has gone wrong at build time.

Subsequent patches will make use of the new helper.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Tested-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Tested-by: Torsten Duwe <duwe@suse.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>

arch/arm64/kernel/module.c

@@ -470,22 +470,39 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 	return -ENOEXEC;
 }
 
-int module_finalize(const Elf_Ehdr *hdr,
+static const Elf_Shdr *find_section(const Elf_Ehdr *hdr,
 				    const Elf_Shdr *sechdrs,
-		    struct module *me)
+				    const char *name)
 {
 	const Elf_Shdr *s, *se;
 	const char *secstrs = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
 
 	for (s = sechdrs, se = sechdrs + hdr->e_shnum; s < se; s++) {
-		if (strcmp(".altinstructions", secstrs + s->sh_name) == 0)
+		if (strcmp(name, secstrs + s->sh_name) == 0)
+			return s;
+	}
+
+	return NULL;
+}
+
+int module_finalize(const Elf_Ehdr *hdr,
+		    const Elf_Shdr *sechdrs,
+		    struct module *me)
+{
+	const Elf_Shdr *s;
+
+	s = find_section(hdr, sechdrs, ".altinstructions");
+	if (s)
 		apply_alternatives_module((void *)s->sh_addr, s->sh_size);
+
 #ifdef CONFIG_ARM64_MODULE_PLTS
-		if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
-		    !strcmp(".text.ftrace_trampoline", secstrs + s->sh_name))
+	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE)) {
+		s = find_section(hdr, sechdrs, ".text.ftrace_trampoline");
+		if (!s)
+			return -ENOEXEC;
 		me->arch.ftrace_trampoline = (void *)s->sh_addr;
-#endif
 	}
+#endif
 
 	return 0;
 }