Patch "module: keep percpu symbols in module's symtab" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

module: keep percpu symbols in module's symtab

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
module-keep-percpu-symbols-in-module-s-symtab.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Miroslav Benes <[email protected]>
Date: Thu, 26 Nov 2015 13:18:06 +1030
Subject: module: keep percpu symbols in module's symtab

From: Miroslav Benes <[email protected]>

commit e0224418516b4d8a6c2160574bac18447c354ef0 upstream.

Currently, percpu symbols from .data..percpu ELF section of a module are not copied over and stored in final symtab array of struct module. Consequently such symbol cannot be returned via kallsyms API (for example kallsyms_lookup_name). This can be especially confusing when the percpu symbol is exported. Only its __ksymtab et al. are present in its symtab.

The culprit is in layout_and_allocate() function where SHF_ALLOC flag is dropped for .data..percpu section. There is in fact no need to copy the section to final struct module, because kernel module loader allocates extra percpu section by itself. Unfortunately only symbols from SHF_ALLOC sections are copied due to a check in is_core_symbol().

The patch changes is_core_symbol() function to copy over also percpu symbols (their st_shndx points to .data..percpu ELF section). We do it only if CONFIG_KALLSYMS_ALL is set to be consistent with the rest of the function (ELF section is SHF_ALLOC but !SHF_EXECINSTR). Finally elf_type() returns type 'a' for a percpu symbol because its address is absolute.

---
 kernel/module.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

@@ -2404,7 +2404,7 @@ static char elf_type(const Elf_Sym *sym,
 	}
 	if (sym->st_shndx == SHN_UNDEF)
 		return 'U';
-	if (sym->st_shndx == SHN_ABS)
+	if (sym->st_shndx == SHN_ABS || sym->st_shndx == info->index.pcpu)
 		return 'a';
 	if (sym->st_shndx >= SHN_LORESERVE)
 		return '?';

@@ -2433,7 +2433,7 @@ static char elf_type(const Elf_Sym *sym,
 }
 
 static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
-			unsigned int shnum)
+			unsigned int shnum, unsigned int pcpundx)
 {
 	const Elf_Shdr *sec;
 

@@ -2442,6 +2442,11 @@ static bool is_core_symbol(const Elf_Sym
 	    || !src->st_name)
 		return false;
 
+#ifdef CONFIG_KALLSYMS_ALL
+	if (src->st_shndx == pcpundx)
+		return true;
+#endif
+
 	sec = sechdrs + src->st_shndx;
 	if (!(sec->sh_flags & SHF_ALLOC)
 #ifndef CONFIG_KALLSYMS_ALL

@@ -2479,7 +2484,8 @@ static void layout_symtab(struct module
 	/* Compute total space required for the core symbols' strtab. */
 	for (ndst = i = 0; i < nsrc; i++) {
 		if (i == 0 ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
+		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
+				   info->index.pcpu)) {
 			strtab_size += strlen(&info->strtab[src[i].st_name])+1;
 			ndst++;
 		}

@@ -2537,7 +2543,8 @@ static void add_kallsyms(struct module *
 	src = mod->kallsyms->symtab;
 	for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
 		if (i == 0 ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
+		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
+				   info->index.pcpu)) {
 			dst[ndst] = src[i];
 			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
 			s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/module-keep-percpu-symbols-in-module-s-symtab.patch