MINOR: sym: also read the compressed symbol tables from the ELF files

Distributions ship libraries stripped, keeping local symbols in a
compressed .gnu_debugdata section. It is not loaded in memory, so it was
only reachable for objects mapping it in a segment of their own, i.e. the
ones we build, leaving system libraries unnamed.

Look for that section in the ELF files already mapped to read their
.symtab, and hand it to the same decompressor. A separate debug file still
wins as it is more complete, and the file mappings are released right away
since names point into the decompressed image. On Fedora this names
libcrypto, libssl, liblua, libm, libcrypt and libz, turning
"liblua-5:luaopen_package+0x41a0/0x205" into "liblua-5:luaV_execute+0x90f".

diff --git a/src/sym.c b/src/sym.c
index 1b00f39fa80538bfd8f428bc44544e27e0e07a89..2681a905776caed3023dbf223ff56e482fc2cd96 100644 (file)
--- a/src/sym.c
+++ b/src/sym.c
@@ -648,6 +648,28 @@ static void *map_debug_file(const char *obj_path, size_t *dsz,
        return NULL;
 }
 
+static const void *elf_find_section(const void *map, size_t sz, const ElfW(Ehdr) *eh,
+                                   const char *shstr, size_t shstrsz,
+                                   const char *name, size_t *psize)
+{
+       const ElfW(Shdr) *sh = (const ElfW(Shdr) *)((const char *)map + eh->e_shoff);
+       const void *data;
+       unsigned int i;
+
+       for (i = 0; i < eh->e_shnum; i++) {
+               if (sh[i].sh_type == SHT_NOBITS || sh[i].sh_name >= shstrsz)
+                       continue;
+               if (strcmp(shstr + sh[i].sh_name, name) != 0)
+                       continue;
+
+               data = (const char *)map + sh[i].sh_offset;
+               if (!in_map(data, sh[i].sh_size, map, sz) || !sh[i].sh_size)
+                       return NULL;
+               *psize = sh[i].sh_size;
+               return data;
+       }
+       return NULL;
+}
 
 static void elf_find_symtab(const ElfW(Ehdr) *eh,
                            const ElfW(Shdr) **symtab, const ElfW(Shdr) **strtab)
@@ -667,12 +689,6 @@ static void elf_find_symtab(const ElfW(Ehdr) *eh,
 
 /* Decompresses the xz-compressed ELF image
  */
-#if !defined(HA_HAVE_DL_ITERATE_PHDR)
-/* the loader callback is its only caller here; the file parsing which also
- * needs it comes next, and is itself kept compiled without the API.
- */
-__maybe_unused
-#endif
 static void collect_debugdata(struct sym_build *b, const struct sym_obj *obj,
                              const unsigned char *blob, size_t size)
 {
@@ -745,7 +761,6 @@ static void collect_debugdata(struct sym_build *b, const struct sym_obj *obj,
        xz_dec_end(dec);
 }
 
-
 /* Returns the build-id, if available */
 static const unsigned char *mem_build_id(const ElfW(Phdr) *phdr, unsigned int phnum,
                                         unsigned long base, size_t *len)
@@ -817,6 +832,8 @@ static int parse_object(struct sym_build *b, const char *path, const struct sym_
        const ElfW(Ehdr) *eh, *deh = NULL;
        void *map, *dmap = NULL;
        size_t mapsz = 0, dmapsz = 0;
+       const void *blob = NULL;
+       size_t blobsz = 0;
        const unsigned char *bid = NULL, *mbid;
        const char *link = NULL, *shstr;
        size_t bidlen = 0, mbidlen, linklen = 0, linksz = 0, shstrsz = 0;
@@ -855,8 +872,13 @@ static int parse_object(struct sym_build *b, const char *path, const struct sym_
 
        elf_find_symtab(eh, &symtab, &strtab);
 
-       /* if the object is stripped (no symtab), try separate debug info */
+       /* if the object is stripped (no symtab), try separate debug info, then
+        * the compressed table it may carry.
+        */
        if (!symtab) {
+               if (shstr)
+                       blob = elf_find_section(map, mapsz, eh, shstr, shstrsz,
+                                               ".gnu_debugdata", &blobsz);
                dmap = map_debug_file(path, &dmapsz, bid, bidlen, link, linklen, linksz);
                if (dmap) {
                        /* the debug file's symtab matches the object's vaddrs */
@@ -865,17 +887,30 @@ static int parse_object(struct sym_build *b, const char *path, const struct sym_
                }
        }
 
-       if (!symtab) {
+       before = b->n;
+
+       if (symtab) {
+               collect_syms(b, deh ? dmap : map, deh ? dmapsz : mapsz,
+                            symtab, strtab, obj->base, obj);
+       }
+       else if (blob) {
+               /* not loaded in memory by this object, but readable here. The
+                * collected names point into the decompressed image, which the
+                * builder owns, so the file mappings are not needed afterwards.
+                */
+               collect_debugdata(b, obj, blob, blobsz);
+               munmap(map, mapsz);
+               if (dmap)
+                       munmap(dmap, dmapsz);
+               return b->oom ? -1 : (b->n != before);
+       }
+       else {
                munmap(map, mapsz);
                if (dmap)
                        munmap(dmap, dmapsz);
                return 0;
        }
 
-       before = b->n;
-       collect_syms(b, deh ? dmap : map, deh ? dmapsz : mapsz,
-                    symtab, strtab, obj->base, obj);
-
        if (b->n == before) {
                munmap(map, mapsz);
                if (dmap)