kbuild: check for section mismatch during modpost stage

Section mismatch is identified as references to .init*
sections from non .init sections. And likewise references
to .exit.* sections outside .exit sections.

.init.* sections are discarded after a module is initialized
and references to .init.* sections are oops candidates.
.exit.* sections are discarded when a module is built-in and
thus references to .exit are also oops candidates.

The checks were possible to do using 'make buildcheck' which
called the two perl scripts: reference_discarded.pl and
reference_init.pl. This patch just moves the same functionality
inside modpost and the scripts are then obsoleted.
They will though be kept for a while so users can do double
checks - but note that some .o files are skipped by the perl scripts
so result is not 1:1.
All credit for the concept goes to Keith Owens who implemented
the original perl scrips - this patch just moves it to modpost.

Compared to the perl script the implmentation in modpost will be run
for each kernel build - thus catching the error much sooner, but
the downside is that the individual .o file are not always identified.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mod/modpost.c

@@ -451,6 +451,262 @@ static char *get_modinfo(void *modinfo, unsigned long modinfo_len,
 	return NULL;
 }
 
+/*
+ * Find symbols before or equal addr and after addr - in the section sec
+ **/
+static void find_symbols_between(struct elf_info *elf, Elf_Addr addr,
+				 const char *sec,
+			         Elf_Sym **before, Elf_Sym **after)
+{
+	Elf_Sym *sym;
+	Elf_Ehdr *hdr = elf->hdr;
+	Elf_Addr beforediff = ~0;
+	Elf_Addr afterdiff = ~0;
+	const char *secstrings = (void *)hdr +
+				 elf->sechdrs[hdr->e_shstrndx].sh_offset;
+
+	*before = NULL;
+	*after = NULL;
+
+	for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
+		const char *symsec;
+
+		if (sym->st_shndx >= SHN_LORESERVE)
+			continue;
+		symsec = secstrings + elf->sechdrs[sym->st_shndx].sh_name;
+		if (strcmp(symsec, sec) != 0)
+			continue;
+		if (sym->st_value <= addr) {
+			if ((addr - sym->st_value) < beforediff) {
+				beforediff = addr - sym->st_value;
+				*before = sym;
+			}
+		}
+		else
+		{
+			if ((sym->st_value - addr) < afterdiff) {
+				afterdiff = sym->st_value - addr;
+				*after = sym;
+			}
+		}
+	}
+}
+
+/**
+ * Print a warning about a section mismatch.
+ * Try to find symbols near it so user can find it.
+ **/
+static void warn_sec_mismatch(const char *modname, const char *fromsec,
+			      struct elf_info *elf, Elf_Sym *sym, Elf_Rela r)
+{
+	Elf_Sym *before;
+	Elf_Sym *after;
+	Elf_Ehdr *hdr = elf->hdr;
+	Elf_Shdr *sechdrs = elf->sechdrs;
+	const char *secstrings = (void *)hdr +
+				 sechdrs[hdr->e_shstrndx].sh_offset;
+	const char *secname = secstrings + sechdrs[sym->st_shndx].sh_name;
+
+	find_symbols_between(elf, r.r_offset, fromsec, &before, &after);
+
+	if (before && after) {
+		warn("%s - Section mismatch: reference to %s from %s "
+		     "between '%s' (at offset 0x%lx) and '%s'\n",
+		     modname, secname, fromsec,
+		     elf->strtab + before->st_name,
+		     (long)(r.r_offset - before->st_value),
+		     elf->strtab + after->st_name);
+	} else if (before) {
+		warn("%s - Section mismatch: reference to %s from %s "
+		     "after '%s' (at offset 0x%lx)\n",
+		     modname, secname, fromsec, 
+		     elf->strtab + before->st_name,
+		     (long)(r.r_offset - before->st_value));
+	} else if (after) {
+		warn("%s - Section mismatch: reference to %s from %s "
+		     "before '%s' (at offset -0x%lx)\n",
+		     modname, secname, fromsec, 
+		     elf->strtab + before->st_name,
+		     (long)(before->st_value - r.r_offset));
+	} else {
+		warn("%s - Section mismatch: reference to %s from %s "
+		     "(offset 0x%lx)\n",
+		     modname, secname, fromsec, (long)r.r_offset);
+	}
+}
+
+/**
+ * A module includes a number of sections that are discarded
+ * either when loaded or when used as built-in.
+ * For loaded modules all functions marked __init and all data
+ * marked __initdata will be discarded when the module has been intialized.
+ * Likewise for modules used built-in the sections marked __exit
+ * are discarded because __exit marked function are supposed to be called
+ * only when a moduel is unloaded which never happes for built-in modules.
+ * The check_sec_ref() function traverses all relocation records
+ * to find all references to a section that reference a section that will
+ * be discarded and warns about it.
+ **/
+static void check_sec_ref(struct module *mod, const char *modname,
+			  struct elf_info *elf,
+			  int section(const char*),
+			  int section_ref_ok(const char *))
+{
+	int i;
+	Elf_Sym  *sym;
+	Elf_Ehdr *hdr = elf->hdr;
+	Elf_Shdr *sechdrs = elf->sechdrs;
+	const char *secstrings = (void *)hdr +
+				 sechdrs[hdr->e_shstrndx].sh_offset;
+
+	/* Walk through all sections */
+	for (i = 0; i < hdr->e_shnum; i++) {
+		const char *name = secstrings + sechdrs[i].sh_name +
+						strlen(".rela");
+		/* We want to process only relocation sections and not .init */
+		if (section_ref_ok(name) || (sechdrs[i].sh_type != SHT_RELA))
+			continue;
+		Elf_Rela *rela;
+		Elf_Rela *start = (void *)hdr + sechdrs[i].sh_offset;
+		Elf_Rela *stop  = (void*)start + sechdrs[i].sh_size;
+
+		for (rela = start; rela < stop; rela++) {
+			Elf_Rela r;
+			const char *secname;
+			r.r_offset = TO_NATIVE(rela->r_offset);
+			r.r_info   = TO_NATIVE(rela->r_info);
+			sym = elf->symtab_start + ELF_R_SYM(r.r_info);
+			secname = secstrings + sechdrs[sym->st_shndx].sh_name;
+			/* Skip special sections */
+			if (sym->st_shndx >= SHN_LORESERVE)
+				continue;
+
+			if (section(secname))
+				warn_sec_mismatch(modname, name, elf, sym, r);
+		}
+	}
+}
+
+/**
+ * Functions used only during module init is marked __init and is stored in
+ * a .init.text section. Likewise data is marked __initdata and stored in
+ * a .init.data section.
+ * If this section is one of these sections return 1
+ * See include/linux/init.h for the details
+ **/
+static int init_section(const char *name)
+{
+	if (strcmp(name, ".init") == 0)
+		return 1;
+	if (strncmp(name, ".init.", strlen(".init.")) == 0)
+		return 1;
+	return 0;
+}
+
+/**
+ * Identify sections from which references to a .init section is OK.
+ * 
+ * Unfortunately references to read only data that referenced .init
+ * sections had to be excluded. Almost all of these are false
+ * positives, they are created by gcc. The downside of excluding rodata
+ * is that there really are some user references from rodata to
+ * init code, e.g. drivers/video/vgacon.c:
+ * 
+ * const struct consw vga_con = {
+ *        con_startup:            vgacon_startup,
+ *
+ * where vgacon_startup is __init.  If you want to wade through the false
+ * positives, take out the check for rodata.
+ **/
+static int init_section_ref_ok(const char *name)
+{
+	const char **s;
+	/* Absolute section names */
+	const char *namelist1[] = {
+		".init",
+		".stab",
+		".rodata",
+		".text.lock",
+		".pci_fixup_header",
+		".pci_fixup_final",
+		".pdr",
+		"__param",
+		NULL
+	};
+	/* Start of section names */
+	const char *namelist2[] = {
+		".init.",
+		".altinstructions",
+		".eh_frame",
+		".debug",
+		NULL
+	};
+
+	for (s = namelist1; *s; s++)
+		if (strcmp(*s, name) == 0)
+			return 1;
+	for (s = namelist2; *s; s++)	
+		if (strncmp(*s, name, strlen(*s)) == 0)
+			return 1;
+	return 0;
+}
+
+/*
+ * Functions used only during module exit is marked __exit and is stored in
+ * a .exit.text section. Likewise data is marked __exitdata and stored in
+ * a .exit.data section.
+ * If this section is one of these sections return 1
+ * See include/linux/init.h for the details
+ **/
+static int exit_section(const char *name)
+{
+	if (strcmp(name, ".exit.text") == 0)
+		return 1;
+	if (strcmp(name, ".exit.data") == 0)
+		return 1;
+	return 0;
+
+}
+
+/*
+ * Identify sections from which references to a .exit section is OK.
+ * 
+ * [OPD] Keith Ownes <kaos@sgi.com> commented:
+ * For our future {in}sanity, add a comment that this is the ppc .opd
+ * section, not the ia64 .opd section.
+ * ia64 .opd should not point to discarded sections.
+ **/
+static int exit_section_ref_ok(const char *name)
+{
+	const char **s;
+	/* Absolute section names */
+	const char *namelist1[] = {
+		".exit.text",
+		".exit.data",
+		".init.text",
+		".opd", /* See comment [OPD] */
+		".altinstructions",
+		".pdr",
+		".exitcall.exit",
+		".eh_frame",
+		".stab",
+		NULL
+	};
+	/* Start of section names */
+	const char *namelist2[] = {
+		".debug",
+		NULL
+	};
+
+	for (s = namelist1; *s; s++)
+		if (strcmp(*s, name) == 0)
+			return 1;
+	for (s = namelist2; *s; s++)	
+		if (strncmp(*s, name, strlen(*s)) == 0)
+			return 1;
+	return 0;
+}
+
 static void read_symbols(char *modname)
 {
 	const char *symname;
@@ -476,6 +732,8 @@ static void read_symbols(char *modname)
 		handle_modversions(mod, &info, sym, symname);
 		handle_moddevtable(mod, &info, sym, symname);
 	}
+	check_sec_ref(mod, modname, &info, init_section, init_section_ref_ok);
+	check_sec_ref(mod, modname, &info, exit_section, exit_section_ref_ok);
 
 	version = get_modinfo(info.modinfo, info.modinfo_len, "version");
 	if (version)

scripts/mod/modpost.h

@@ -16,17 +16,27 @@
 #define Elf_Ehdr    Elf32_Ehdr 
 #define Elf_Shdr    Elf32_Shdr 
 #define Elf_Sym     Elf32_Sym
+#define Elf_Addr    Elf32_Addr
+#define Elf_Section Elf32_Section
 #define ELF_ST_BIND ELF32_ST_BIND
 #define ELF_ST_TYPE ELF32_ST_TYPE
 
+#define Elf_Rela    Elf32_Rela
+#define ELF_R_SYM   ELF32_R_SYM
+#define ELF_R_TYPE  ELF32_R_TYPE
 #else
 
 #define Elf_Ehdr    Elf64_Ehdr 
 #define Elf_Shdr    Elf64_Shdr 
 #define Elf_Sym     Elf64_Sym
+#define Elf_Addr    Elf64_Addr
+#define Elf_Section Elf64_Section
 #define ELF_ST_BIND ELF64_ST_BIND
 #define ELF_ST_TYPE ELF64_ST_TYPE
 
+#define Elf_Rela    Elf64_Rela
+#define ELF_R_SYM   ELF64_R_SYM
+#define ELF_R_TYPE  ELF64_R_TYPE
 #endif
 
 #if KERNEL_ELFDATA != HOST_ELFDATA