module: reduce symbol table for loaded modules (v2)

Discard all symbols not interesting for kallsyms use: absolute,
section, and in the common case (!KALLSYMS_ALL) data ones.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 94 insertions, 7 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index 1c755b2f937d..1d3ccb173fd6 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -308,9 +308,13 @@ struct module
 #endif
 
 #ifdef CONFIG_KALLSYMS
-        /* We keep the symbol and string tables for kallsyms. */
-        Elf_Sym *symtab;
-        unsigned int num_symtab;
+        /*
+         * We keep the symbol and string tables for kallsyms.
+         * The core_* fields below are temporary, loader-only (they
+         * could really be discarded after module init).
+         */
+        Elf_Sym *symtab, *core_symtab;
+        unsigned int num_symtab, core_num_syms;
         char *strtab;
 
         /* Section attributes */
diff --git a/kernel/module.c b/kernel/module.c
index e6bc4b28aa62..97f4d5e15535 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1862,13 +1862,68 @@ static char elf_type(const Elf_Sym *sym,
         return '?';
 }
 
+static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
+                           unsigned int shnum)
+{
+        const Elf_Shdr *sec;
+
+        if (src->st_shndx == SHN_UNDEF
+            || src->st_shndx >= shnum
+            || !src->st_name)
+                return false;
+
+        sec = sechdrs + src->st_shndx;
+        if (!(sec->sh_flags & SHF_ALLOC)
+#ifndef CONFIG_KALLSYMS_ALL
+            || !(sec->sh_flags & SHF_EXECINSTR)
+#endif
+            || (sec->sh_entsize & INIT_OFFSET_MASK))
+                return false;
+
+        return true;
+}
+
+static unsigned long layout_symtab(struct module *mod,
+                                   Elf_Shdr *sechdrs,
+                                   unsigned int symindex,
+                                   const Elf_Ehdr *hdr,
+                                   const char *secstrings)
+{
+        unsigned long symoffs;
+        Elf_Shdr *symsect = sechdrs + symindex;
+        const Elf_Sym *src;
+        unsigned int i, nsrc, ndst;
+
+        /* Put symbol section at end of init part of module. */
+        symsect->sh_flags |= SHF_ALLOC;
+        symsect->sh_entsize = get_offset(mod, &mod->init_size, symsect,
+                                         symindex) | INIT_OFFSET_MASK;
+        DEBUGP("\t%s\n", secstrings + symsect->sh_name);
+
+        src = (void *)hdr + symsect->sh_offset;
+        nsrc = symsect->sh_size / sizeof(*src);
+        for (ndst = i = 1; i < nsrc; ++i, ++src)
+                if (is_core_symbol(src, sechdrs, hdr->e_shnum))
+                        ++ndst;
+
+        /* Append room for core symbols at end of core part. */
+        symoffs = ALIGN(mod->core_size, symsect->sh_addralign ?: 1);
+        mod->core_size = symoffs + ndst * sizeof(Elf_Sym);
+
+        return symoffs;
+}
+
 static void add_kallsyms(struct module *mod,
                          Elf_Shdr *sechdrs,
+                         unsigned int shnum,
                          unsigned int symindex,
                          unsigned int strindex,
+                         unsigned long symoffs,
                          const char *secstrings)
 {
-        unsigned int i;
+        unsigned int i, ndst;
+        const Elf_Sym *src;
+        Elf_Sym *dst;
 
         mod->symtab = (void *)sechdrs[symindex].sh_addr;
         mod->num_symtab = sechdrs[symindex].sh_size / sizeof(Elf_Sym);
@@ -1878,12 +1933,32 @@ static void add_kallsyms(struct module *mod,
         for (i = 0; i < mod->num_symtab; i++)
                 mod->symtab[i].st_info
                         = elf_type(&mod->symtab[i], sechdrs, secstrings, mod);
+
+        mod->core_symtab = dst = mod->module_core + symoffs;
+        src = mod->symtab;
+        *dst = *src;
+        for (ndst = i = 1; i < mod->num_symtab; ++i, ++src) {
+                if (!is_core_symbol(src, sechdrs, shnum))
+                        continue;
+                dst[ndst] = *src;
+                ++ndst;
+        }
+        mod->core_num_syms = ndst;
 }
 #else
+static inline unsigned long layout_symtab(struct module *mod,
+                                          Elf_Shdr *sechdrs,
+                                          unsigned int symindex,
+                                          const Elf_Hdr *hdr,
+                                          const char *secstrings)
+{
+}
 static inline void add_kallsyms(struct module *mod,
                                 Elf_Shdr *sechdrs,
+                                unsigned int shnum,
                                 unsigned int symindex,
                                 unsigned int strindex,
+                                unsigned long symoffs,
                                 const char *secstrings)
 {
 }
@@ -1959,6 +2034,9 @@ static noinline struct module *load_module(void __user *umod,
         struct module *mod;
         long err = 0;
         void *percpu = NULL, *ptr = NULL; /* Stops spurious gcc warning */
+#ifdef CONFIG_KALLSYMS
+        unsigned long symoffs;
+#endif
         mm_segment_t old_fs;
 
         DEBUGP("load_module: umod=%p, len=%lu, uargs=%p\n",
@@ -2041,8 +2119,7 @@ static noinline struct module *load_module(void __user *umod,
         sechdrs[infoindex].sh_flags &= ~(unsigned long)SHF_ALLOC;
         sechdrs[versindex].sh_flags &= ~(unsigned long)SHF_ALLOC;
 #ifdef CONFIG_KALLSYMS
-        /* Keep symbol and string tables for decoding later. */
-        sechdrs[symindex].sh_flags |= SHF_ALLOC;
+        /* Keep string table for decoding later. */
         sechdrs[strindex].sh_flags |= SHF_ALLOC;
 #endif
 
@@ -2109,6 +2186,7 @@ static noinline struct module *load_module(void __user *umod,
            this is done generically; there doesn't appear to be any
            special cases for the architectures. */
         layout_sections(mod, hdr, sechdrs, secstrings);
+        symoffs = layout_symtab(mod, sechdrs, symindex, hdr, secstrings);
 
         /* Do the allocs. */
         ptr = module_alloc_update_bounds(mod->core_size);
@@ -2313,7 +2391,8 @@ static noinline struct module *load_module(void __user *umod,
         percpu_modcopy(mod->percpu, (void *)sechdrs[pcpuindex].sh_addr,
                        sechdrs[pcpuindex].sh_size);
 
-        add_kallsyms(mod, sechdrs, symindex, strindex, secstrings);
+        add_kallsyms(mod, sechdrs, hdr->e_shnum, symindex, strindex,
+                     symoffs, secstrings);
 
         if (!mod->taints) {
                 struct _ddebug *debug;
@@ -2491,6 +2570,10 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
         /* Drop initial reference. */
         module_put(mod);
         trim_init_extable(mod);
+#ifdef CONFIG_KALLSYMS
+        mod->num_symtab = mod->core_num_syms;
+        mod->symtab = mod->core_symtab;
+#endif
         module_free(mod, mod->module_init);
         mod->module_init = NULL;
         mod->init_size = 0;