[PATCH v2 17/26] modpost: extract symbol versions from *.cmd files

Masahiro Yamada
2022-05-01T08:43:58Z

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 |---------| |------------|
$(CC) $(LD) | | | final link |
*.c ------> *.o --------> *.o -->| modpost |-- *.o ------>| for |
/ | |-- *.mod.c -->| vmlinux |
genksyms / | | | or 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 embed CRCs at the very
last moment.

New implementation
==================

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

We can pass the symbol versions to modpost as separate text data.
(The previous commit output the versions in *.cmd files.)

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

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

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

scripts/mod/modpost.c | 184 +++++++++++++++++++++++++++++++-----------
1 file changed, 136 insertions(+), 48 deletions(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 375b9463cb8a..d8df0f8d3def 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -429,19 +429,10 @@ 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)
@@ -664,33 +655,6 @@ 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)
{
@@ -1997,6 +1961,111 @@ 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_from_cmdfile(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) {
+ error("invalid\n");
+ goto out;
+ }
+
+ namelen = p - name;
+
+ p++;
+
+ if (!isdigit(*p)) {
+ error("invalid\n");
+ goto out;
+ }
+
+ crc = strtol(p, &p, 0);
+
+ if (*p != '\n') {
+ error("invalid\n");
+ goto out;
+ }
+
+ name[namelen] = '\0';
+
+ sym = sym_find_with_module(name, mod);
+ if (!sym) {
+ warn("Skip the version for unexported symbol \"%s\" [%s%s]",
+ name, mod->name, mod->is_vmlinux ? "" : ".ko");
+ continue;
+ }
+ sym_set_crc(sym, crc);
+ }
+
+out:
+ 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_from_cmdfile(obj, mod);
+
+ free(buf);
+}
+
static void read_symbols(const char *modname)
{
const char *symname;
@@ -2057,9 +2126,6 @@ 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
@@ -2088,12 +2154,17 @@ 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)
@@ -2453,7 +2524,7 @@ static void read_dump(const char *fname)
}
s = sym_add_exported(symname, mod, export_no(export));
s->is_static = false;
- sym_set_crc(symname, crc);
+ sym_set_crc(s, crc);
sym_update_namespace(symname, namespace);
}
free(buf);
@@ -2483,6 +2554,20 @@ static void write_dump(const char *fname)
free(buf.p);
}

+static void check_symversions(struct module *mod)
+{
+ struct symbol *sym;
+
+ 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);
+ }
+ }
+}
+
static void write_namespace_deps_files(const char *fname)
{
struct module *mod;
@@ -2579,6 +2664,9 @@ int main(int argc, char **argv)
char fname[PATH_MAX];
int ret;

+ if (modversions && !mod->from_dump)
+ check_symversions(mod);
+
if (mod->is_vmlinux || mod->from_dump)
continue;

--
2.32.0