[PATCH v3 07/15] modpost: extract symbol versions from *.cmd files

Masahiro Yamada
2022-05-05T07:27:40Z

Currently, CONFIG_MODVERSIONS needs extra link to embed the symbol
versions into ELF objects. Then, modpost extracts the version CRCs
from them.

The following figures show how it currently works, and how I am trying
to change it.

Current implementation
======================
|----------|
embed CRC -------------------------->| final |
$(CC) $(LD) / |---------| | link for |
*.c ------> *.o -------> *.o -->| modpost | | vmlinux |
/ | |-- *.mod.c -->| or |
genksyms / |---------| | module |
*.c ------> *.symversions |----------|

Genksyms outputs the calculated CRCs in the form of linker script
(*.symversions), which is used by $(LD) to update the object.

If CONFIG_LTO_CLANG=y, the build process becomes much more complex.
Embedding the CRCs is postponed until the LLVM bitcode is converted
into ELF, creating another intermediate *.prelink.o.

However, this complexity is unneeded. There is no reason why we must
embed version CRCs in objects so early.

There is final link stage for vmlinux (scripts/link-vmlinux.sh) and
modules (scripts/Makefile.modfinal). We can link CRCs at the very last
moment.

New implementation
==================
|----------|
--------------------------------------->| final |
$(CC) / |---------| | link for |
*.c ------> *.o ---->| | | vmlinux |
| modpost |--- .vmlinux-symver.c -->| or |
genksyms | |--- *.mod.c ------------>| module |
*.c ------> *.cmd -->|---------| |----------|

Pass the symbol versions to modpost as separate text data, which are
available in *.cmd files.

This commit changes modpost to extract CRCs from *.cmd files instead of
from ELF objects.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

(no changes since v2)

Changes in v2:
- Simplify the implementation (parse .cmd files after ELF)

scripts/mod/modpost.c | 177 ++++++++++++++++++++++++++++++------------
1 file changed, 129 insertions(+), 48 deletions(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 78a7107fcc40..92ee1f454e29 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -383,19 +383,10 @@ static struct symbol *sym_add_exported(const char *name, struct module *mod,
return s;
}

-static void sym_set_crc(const char *name, unsigned int crc)
+static void sym_set_crc(struct symbol *sym, unsigned int crc)
{
- struct symbol *s = find_symbol(name);
-
- /*
- * Ignore stand-alone __crc_*, which might be auto-generated symbols
- * such as __*_veneer in ARM ELF.
- */
- if (!s)
- return;
-
- s->crc = crc;
- s->crc_valid = true;
+ sym->crc = crc;
+ sym->crc_valid = true;
}

static void *grab_file(const char *filename, size_t *size)
@@ -618,33 +609,6 @@ static int ignore_undef_symbol(struct elf_info *info, const char *symname)
return 0;
}

-static void handle_modversion(const struct module *mod,
- const struct elf_info *info,
- const Elf_Sym *sym, const char *symname)
-{
- unsigned int crc;
-
- if (sym->st_shndx == SHN_UNDEF) {
- warn("EXPORT symbol \"%s\" [%s%s] version generation failed, symbol will not be versioned.\n"
- "Is \"%s\" prototyped in <asm/asm-prototypes.h>?\n",
- symname, mod->name, mod->is_vmlinux ? "" : ".ko",
- symname);
-
- return;
- }
-
- if (sym->st_shndx == SHN_ABS) {
- crc = sym->st_value;
- } else {
- unsigned int *crcp;
-
- /* symbol points to the CRC in the ELF object */
- crcp = sym_get_data(info, sym);
- crc = TO_NATIVE(*crcp);
- }
- sym_set_crc(symname, crc);
-}
-
static void handle_symbol(struct module *mod, struct elf_info *info,
const Elf_Sym *sym, const char *symname)
{
@@ -1955,6 +1919,102 @@ static char *remove_dot(char *s)
return s;
}

+/*
+ * The CRCs are recorded in .*.cmd files in the form of:
+ * #SYMVER <name> <crc>
+ */
+static void extract_crcs_for_object(const char *object, struct module *mod)
+{
+ char cmd_file[PATH_MAX];
+ char *buf, *p;
+ const char *base;
+ int dirlen, ret;
+
+ base = strrchr(object, '/');
+ if (base) {
+ base++;
+ dirlen = base - object;
+ } else {
+ dirlen = 0;
+ base = object;
+ }
+
+ ret = snprintf(cmd_file, sizeof(cmd_file), "%.*s.%s.cmd",
+ dirlen, object, base);
+ if (ret >= sizeof(cmd_file)) {
+ error("%s: too long path was truncated\n", cmd_file);
+ return;
+ }
+
+ buf = read_text_file(cmd_file);
+ p = buf;
+
+ while ((p = strstr(p, "\n#SYMVER "))) {
+ char *name;
+ size_t namelen;
+ unsigned int crc;
+ struct symbol *sym;
+
+ name = p + strlen("\n#SYMVER ");
+
+ p = strchr(name, ' ');
+ if (!p)
+ break;
+
+ namelen = p - name;
+ p++;
+
+ if (!isdigit(*p))
+ continue; /* skip this line */
+
+ crc = strtol(p, &p, 0);
+ if (*p != '\n')
+ continue; /* skip this line */
+
+ name[namelen] = '\0';
+
+ sym = sym_find_with_module(name, mod);
+ if (!sym) {
+ warn("Skip the version for unexported symbol \"%s\" [%s%s]\n",
+ name, mod->name, mod->is_vmlinux ? "" : ".ko");
+ continue;
+ }
+ sym_set_crc(sym, crc);
+ }
+
+ free(buf);
+}
+
+/*
+ * The symbol versions (CRC) are recorded in the .*.cmd files.
+ * Parse them to retrieve CRCs for the current module.
+ */
+static void mod_set_crcs(struct module *mod)
+{
+ char objlist[PATH_MAX];
+ char *buf, *p, *obj;
+ int ret;
+
+ if (mod->is_vmlinux) {
+ strcpy(objlist, ".vmlinux.objs");
+ } else {
+ /* objects for a module are listed in the *.mod file. */
+ ret = snprintf(objlist, sizeof(objlist), "%s.mod", mod->name);
+ if (ret >= sizeof(objlist)) {
+ error("%s: too long path was truncated\n", objlist);
+ return;
+ }
+ }
+
+ buf = read_text_file(objlist);
+ p = buf;
+
+ while ((obj = strsep(&p, "\n")) && obj[0])
+ extract_crcs_for_object(obj, mod);
+
+ free(buf);
+}
+
static void read_symbols(const char *modname)
{
const char *symname;
@@ -2015,9 +2075,6 @@ static void read_symbols(const char *modname)
if (strstarts(symname, "__kstrtabns_"))
sym_update_namespace(symname + strlen("__kstrtabns_"),
sym_get_data(&info, sym));
- if (strstarts(symname, "__crc_"))
- handle_modversion(mod, &info, sym,
- symname + strlen("__crc_"));
}

// check for static EXPORT_SYMBOL_* functions && global vars
@@ -2046,12 +2103,17 @@ static void read_symbols(const char *modname)

parse_elf_finish(&info);

- /* Our trick to get versioning for module struct etc. - it's
- * never passed as an argument to an exported function, so
- * the automatic versioning doesn't pick it up, but it's really
- * important anyhow */
- if (modversions)
+ if (modversions) {
+ /*
+ * Our trick to get versioning for module struct etc. - it's
+ * never passed as an argument to an exported function, so
+ * the automatic versioning doesn't pick it up, but it's really
+ * important anyhow
+ */
sym_add_unresolved("module_layout", mod, false);
+
+ mod_set_crcs(mod);
+ }
}

static void read_symbols_from_files(const char *filename)
@@ -2214,6 +2276,23 @@ static void add_header(struct buffer *b, struct module *mod)
buf_printf(b, "\nMODULE_INFO(staging, \"Y\");\n");
}

+static void check_symversions(struct module *mod)
+{
+ struct symbol *sym;
+
+ if (!modversions)
+ return;
+
+ list_for_each_entry(sym, &mod->exported_symbols, list) {
+ if (!sym->crc_valid) {
+ warn("EXPORT symbol \"%s\" [%s%s] version generation failed, symbol will not be versioned.\n"
+ "Is \"%s\" prototyped in <asm/asm-prototypes.h>?\n",
+ sym->name, mod->name, mod->is_vmlinux ? "" : ".ko",
+ sym->name);
+ }
+ }
+}
+
/**
* Record CRCs for unresolved symbols
**/
@@ -2429,7 +2508,7 @@ static void read_dump(const char *fname)
}
s = sym_add_exported(symname, mod, gpl_only);
s->is_static = false;
- sym_set_crc(symname, crc);
+ sym_set_crc(s, crc);
sym_update_namespace(symname, namespace);
}
free(buf);
@@ -2554,6 +2633,8 @@ int main(int argc, char **argv)
if (mod->from_dump)
continue;

+ check_symversions(mod);
+
if (!mod->is_vmlinux)
write_mod_c_file(mod);
}
--
2.32.0