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

Nicolas Schier
2022-05-05T20:11:27Z

On Thu, May 05, 2022 at 04:22:36PM +0900 Masahiro Yamada wrote:
> 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

Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>