[PATCH v5 4/7] module: use kflagstab instead of *_gpl sections

Siddharth Nayyar
2026-03-26T21:25:30Z

Read kflagstab section for vmlinux and modules to determine whether
kernel symbols are GPL only.

This patch eliminates the need for fragmenting the ksymtab for infering
the value of GPL-only symbol flag, henceforth stop populating *_gpl
versions of the ksymtab and kcrctab in modpost.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
---
include/linux/export-internal.h | 21 ++++++++--------
include/linux/module.h | 1 +
kernel/module/internal.h | 1 +
kernel/module/main.c | 55 ++++++++++++++++++++++-------------------
scripts/mod/modpost.c | 8 +++---
5 files changed, 46 insertions(+), 40 deletions(-)

diff --git a/include/linux/export-internal.h b/include/linux/export-internal.h
index 4123c7592404..726054614752 100644
--- a/include/linux/export-internal.h
+++ b/include/linux/export-internal.h
@@ -37,14 +37,14 @@
* section flag requires it. Use '%progbits' instead of '@progbits' since the
* former apparently works on all arches according to the binutils source.
*/
-#define __KSYMTAB(name, sym, sec, ns) \
+#define __KSYMTAB(name, sym, ns) \
asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1" "\n" \
"__kstrtab_" #name ":" "\n" \
" .asciz \"" #name "\"" "\n" \
"__kstrtabns_" #name ":" "\n" \
" .asciz \"" ns "\"" "\n" \
" .previous" "\n" \
- " .section \"___ksymtab" sec "+" #name "\", \"a\"" "\n" \
+ " .section \"___ksymtab+" #name "\", \"a\"" "\n" \
__KSYM_ALIGN "\n" \
"__ksymtab_" #name ":" "\n" \
__KSYM_REF(sym) "\n" \
@@ -59,15 +59,16 @@
#define KSYM_FUNC(name) name
#endif

-#define KSYMTAB_FUNC(name, sec, ns) __KSYMTAB(name, KSYM_FUNC(name), sec, ns)
-#define KSYMTAB_DATA(name, sec, ns) __KSYMTAB(name, name, sec, ns)
+#define KSYMTAB_FUNC(name, ns) __KSYMTAB(name, KSYM_FUNC(name), ns)
+#define KSYMTAB_DATA(name, ns) __KSYMTAB(name, name, ns)

-#define SYMBOL_CRC(sym, crc, sec) \
- asm(".section \"___kcrctab" sec "+" #sym "\",\"a\"" "\n" \
- ".balign 4" "\n" \
- "__crc_" #sym ":" "\n" \
- ".long " #crc "\n" \
- ".previous" "\n")
+#define SYMBOL_CRC(sym, crc) \
+ asm(" .section \"___kcrctab+" #sym "\",\"a\"" "\n" \
+ " .balign 4" "\n" \
+ "__crc_" #sym ":" "\n" \
+ " .long " #crc "\n" \
+ " .previous" "\n" \
+ )

#define SYMBOL_FLAGS(sym, flags) \
asm(" .section \"___kflagstab+" #sym "\",\"a\"" "\n" \
diff --git a/include/linux/module.h b/include/linux/module.h
index 14f391b186c6..aee3accba73c 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -418,6 +418,7 @@ struct module {
/* Exported symbols */
const struct kernel_symbol *syms;
const u32 *crcs;
+ const u8 *flagstab;
unsigned int num_syms;

#ifdef CONFIG_ARCH_USES_CFI_TRAPS
diff --git a/kernel/module/internal.h b/kernel/module/internal.h
index 618202578b42..69b84510e097 100644
--- a/kernel/module/internal.h
+++ b/kernel/module/internal.h
@@ -57,6 +57,7 @@ extern const struct kernel_symbol __start___ksymtab_gpl[];
extern const struct kernel_symbol __stop___ksymtab_gpl[];
extern const u32 __start___kcrctab[];
extern const u32 __start___kcrctab_gpl[];
+extern const u8 __start___kflagstab[];

#define KMOD_PATH_LEN 256
extern char modprobe_path[];
diff --git a/kernel/module/main.c b/kernel/module/main.c
index c3ce106c70af..d237fa4e0737 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -11,6 +11,7 @@
#include <linux/extable.h>
#include <linux/moduleloader.h>
#include <linux/module_signature.h>
+#include <linux/module_symbol.h>
#include <linux/trace_events.h>
#include <linux/init.h>
#include <linux/kallsyms.h>
@@ -87,7 +88,7 @@ struct mod_tree_root mod_tree __cacheline_aligned = {
struct symsearch {
const struct kernel_symbol *start, *stop;
const u32 *crcs;
- enum mod_license license;
+ const u8 *flagstab;
};

/*
@@ -364,19 +365,21 @@ static bool find_exported_symbol_in_section(const struct symsearch *syms,
struct find_symbol_arg *fsa)
{
struct kernel_symbol *sym;
-
- if (!fsa->gplok && syms->license == GPL_ONLY)
- return false;
+ u8 sym_flags;

sym = bsearch(fsa->name, syms->start, syms->stop - syms->start,
sizeof(struct kernel_symbol), cmp_name);
if (!sym)
return false;

+ sym_flags = *(syms->flagstab + (sym - syms->start));
+ if (!fsa->gplok && (sym_flags & KSYM_FLAG_GPL_ONLY))
+ return false;
+
fsa->owner = owner;
fsa->crc = symversion(syms->crcs, sym - syms->start);
fsa->sym = sym;
- fsa->license = syms->license;
+ fsa->license = (sym_flags & KSYM_FLAG_GPL_ONLY) ? GPL_ONLY : NOT_GPL_ONLY;

return true;
}
@@ -387,36 +390,31 @@ static bool find_exported_symbol_in_section(const struct symsearch *syms,
*/
bool find_symbol(struct find_symbol_arg *fsa)
{
- static const struct symsearch arr[] = {
- { __start___ksymtab, __stop___ksymtab, __start___kcrctab,
- NOT_GPL_ONLY },
- { __start___ksymtab_gpl, __stop___ksymtab_gpl,
- __start___kcrctab_gpl,
- GPL_ONLY },
+ const struct symsearch syms = {
+ .start = __start___ksymtab,
+ .stop = __stop___ksymtab,
+ .crcs = __start___kcrctab,
+ .flagstab = __start___kflagstab,
};
struct module *mod;
- unsigned int i;

- for (i = 0; i < ARRAY_SIZE(arr); i++)
- if (find_exported_symbol_in_section(&arr[i], NULL, fsa))
- return true;
+ if (find_exported_symbol_in_section(&syms, NULL, fsa))
+ return true;

list_for_each_entry_rcu(mod, &modules, list,
lockdep_is_held(&module_mutex)) {
- struct symsearch arr[] = {
- { mod->syms, mod->syms + mod->num_syms, mod->crcs,
- NOT_GPL_ONLY },
- { mod->gpl_syms, mod->gpl_syms + mod->num_gpl_syms,
- mod->gpl_crcs,
- GPL_ONLY },
+ const struct symsearch syms = {
+ .start = mod->syms,
+ .stop = mod->syms + mod->num_syms,
+ .crcs = mod->crcs,
+ .flagstab = mod->flagstab,
};

if (mod->state == MODULE_STATE_UNFORMED)
continue;

- for (i = 0; i < ARRAY_SIZE(arr); i++)
- if (find_exported_symbol_in_section(&arr[i], mod, fsa))
- return true;
+ if (find_exported_symbol_in_section(&syms, mod, fsa))
+ return true;
}

pr_debug("Failed to find symbol %s\n", fsa->name);
@@ -2614,6 +2612,7 @@ static int find_module_sections(struct module *mod, struct load_info *info)
sizeof(*mod->gpl_syms),
&mod->num_gpl_syms);
mod->gpl_crcs = section_addr(info, "__kcrctab_gpl");
+ mod->flagstab = section_addr(info, "__kflagstab");

#ifdef CONFIG_CONSTRUCTORS
mod->ctors = section_objs(info, ".ctors",
@@ -2817,8 +2816,12 @@ static int move_module(struct module *mod, struct load_info *info)
return ret;
}

-static int check_export_symbol_versions(struct module *mod)
+static int check_export_symbol_sections(struct module *mod)
{
+ if (mod->num_syms && !mod->flagstab) {
+ pr_err("%s: no flags for exported symbols\n", mod->name);
+ return -ENOEXEC;
+ }
#ifdef CONFIG_MODVERSIONS
if ((mod->num_syms && !mod->crcs) ||
(mod->num_gpl_syms && !mod->gpl_crcs)) {
@@ -3434,7 +3437,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
if (err)
goto free_unload;

- err = check_export_symbol_versions(mod);
+ err = check_export_symbol_sections(mod);
if (err)
goto free_unload;

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 1d721fe67caf..9d96acce60a8 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1876,9 +1876,9 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
if (trim_unused_exports && !sym->used)
continue;

- buf_printf(buf, "KSYMTAB_%s(%s, \"%s\", \"%s\");\n",
+ buf_printf(buf, "KSYMTAB_%s(%s, \"%s\");\n",
sym->is_func ? "FUNC" : "DATA", sym->name,
- sym->is_gpl_only ? "_gpl" : "", sym->namespace);
+ sym->namespace);

buf_printf(buf, "SYMBOL_FLAGS(%s, 0x%02x);\n",
sym->name, get_symbol_flags(sym));
@@ -1899,8 +1899,8 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
sym->name, mod->name, mod->is_vmlinux ? "" : ".ko",
sym->name);

- buf_printf(buf, "SYMBOL_CRC(%s, 0x%08x, \"%s\");\n",
- sym->name, sym->crc, sym->is_gpl_only ? "_gpl" : "");
+ buf_printf(buf, "SYMBOL_CRC(%s, 0x%08x);\n",
+ sym->name, sym->crc);
}
}

--
2.53.0.1018.g2bb0e51243-goog