[RFC PATCH 00/10] scalable symbol flags with __kflagstab

[RFC PATCH 00/10] scalable symbol flags with __kflagstab

[Siddharth Nayyar]

Hi everyone,

This patch series proposes a new, scalable mechanism to represent
boolean flags for exported kernel symbols.

Problem Statement:

The core architectural issue with kernel symbol flags is our reliance on
splitting the main symbol table, ksymtab. To handle a single boolean
property, such as GPL-only, all exported symbols are split across two
separate tables: __ksymtab and __ksymtab_gpl.

This design forces the module loader to perform a separate search on
each of these tables for every symbol it needs, for vmlinux and for all
previously loaded modules.

This approach is fundamentally not scalable. If we were to introduce a
second flag, we would need four distinct symbol tables. For n boolean
flags, this model requires an exponential growth to 2^n tables,
dramatically increasing complexity.

Another consequence of this fragmentation is degraded performance. For
example, a binary search on the symbol table of vmlinux, that would take
only 14 comparison steps (assuming ~2^14 or 16K symbols) in a unified
table, can require up to 26 steps when spread across two tables
(assuming both tables have ~2^13 symbols). This performance penalty
worsens as more flags are added.

Proposed Solution:

This series introduces a __kflagstab section to store symbol flags in a
dedicated data structure, similar to how CRCs are handled in the
__kcrctab.

The flags for a given symbol in __kflagstab will be located at the same
index as the symbol's entry in __ksymtab and its CRC in __kcrctab. This
design decouples the flags from the symbol table itself, allowing us to
maintain a single, sorted __ksymtab. As a result, the symbol search
remains an efficient, single lookup, regardless of the number of flags
we add in the future.

The motivation for this change comes from the Android kernel, which uses
an additional symbol flag to restrict the use of certain exported
symbols by unsigned modules, thereby enhancing kernel security. This
__kflagstab can be implemented as a bitmap to efficiently manage which
symbols are available for general use versus those restricted to signed
modules only.

Patch Series Overview:

* Patch 1-8: Introduce the __kflagstab, migrate the existing GPL-only
  flag to this new mechanism, and clean up the old __ksymtab_gpl
  infrastructure.
* Patch 9-10: Add a "symbol import protection" flag,
  which disallows unsigned modules from importing symbols marked with
  this flag.

This is an RFC, and I am seeking feedback on the overall approach and
implementation before moving forward.

Thanks,
Siddharth Nayyar

Siddharth Nayyar (10):
  define kernel symbol flags
  linker: add kflagstab section to vmlinux and modules
  modpost: create entries for kflagstab
  module loader: use kflagstab instead of *_gpl sections
  modpost: put all exported symbols in ksymtab section
  module loader: remove references of *_gpl sections
  linker: remove *_gpl sections from vmlinux and modules
  remove references to *_gpl sections in documentation
  modpost: add symbol import protection flag to kflagstab
  module loader: enforce symbol import protection

 Documentation/kbuild/modules.rst  |   6 +-
 include/asm-generic/vmlinux.lds.h |  21 +++----
 include/linux/export-internal.h   |  28 ++++++---
 include/linux/module.h            |   4 +-
 include/linux/module_symbol.h     |   6 ++
 kernel/module/internal.h          |   5 +-
 kernel/module/main.c              | 101 ++++++++++++++----------------
 scripts/mod/modpost.c             |  27 ++++++--
 scripts/module.lds.S              |   3 +-
 9 files changed, 107 insertions(+), 94 deletions(-)

-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 01/10] define kernel symbol flags

[Siddharth Nayyar]

Symbol flags is an enumeration used to represent flags as a bitset, for
example a flag to tell if a symbols GPL only.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/module_symbol.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/module_symbol.h b/include/linux/module_symbol.h
index 77c9895b9ddb..574609aced99 100644
--- a/include/linux/module_symbol.h
+++ b/include/linux/module_symbol.h
@@ -2,6 +2,11 @@
 #ifndef _LINUX_MODULE_SYMBOL_H
 #define _LINUX_MODULE_SYMBOL_H
 
+/* Kernel symbol flags bitset. */
+enum ksym_flags {
+	KSYM_FLAG_GPL_ONLY	= 1 << 0,
+};
+
 /* This ignores the intensely annoying "mapping symbols" found in ELF files. */
 static inline bool is_mapping_symbol(const char *str)
 {
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 02/10] linker: add kflagstab section to vmlinux and modules

[Siddharth Nayyar]

This section will contain read-only kernel symbol flag values in the
form of a 8-bit bitset.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/asm-generic/vmlinux.lds.h | 7 +++++++
 scripts/module.lds.S              | 1 +
 2 files changed, 8 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index ae2d2359b79e..310e2de56211 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -518,6 +518,13 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 		__stop___kcrctab_gpl = .;				\
 	}								\
 									\
+	/* Kernel symbol flags table */					\
+	__kflagstab       : AT(ADDR(__kflagstab) - LOAD_OFFSET) {	\
+		__start___kflagstab = .;				\
+		KEEP(*(SORT(___kflagstab+*)))				\
+		__stop___kflagstab = .;					\
+	}								\
+									\
 	/* Kernel symbol table: strings */				\
         __ksymtab_strings : AT(ADDR(__ksymtab_strings) - LOAD_OFFSET) {	\
 		*(__ksymtab_strings)					\
diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index ee79c41059f3..9a8a3b6d1569 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -23,6 +23,7 @@ SECTIONS {
 	__ksymtab_gpl		0 : ALIGN(8) { *(SORT(___ksymtab_gpl+*)) }
 	__kcrctab		0 : ALIGN(4) { *(SORT(___kcrctab+*)) }
 	__kcrctab_gpl		0 : ALIGN(4) { *(SORT(___kcrctab_gpl+*)) }
+	__kflagstab		0 : ALIGN(1) { *(SORT(___kflagstab+*)) }
 
 	.ctors			0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) }
 	.init_array		0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) }
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 03/10] modpost: create entries for kflagstab

[Siddharth Nayyar]

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/export-internal.h | 7 +++++++
 scripts/mod/modpost.c           | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/export-internal.h b/include/linux/export-internal.h
index d445705ac13c..4123c7592404 100644
--- a/include/linux/export-internal.h
+++ b/include/linux/export-internal.h
@@ -69,4 +69,11 @@
 	    ".long " #crc					"\n" \
 	    ".previous"						"\n")
 
+#define SYMBOL_FLAGS(sym, flags)					\
+	asm("	.section \"___kflagstab+" #sym "\",\"a\""	"\n"	\
+	    "__flags_" #sym ":"					"\n"	\
+	    "	.byte " #flags					"\n"	\
+	    "	.previous"					"\n"	\
+	)
+
 #endif /* __LINUX_EXPORT_INTERNAL_H__ */
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 5ca7c268294e..f5ce7aeed52d 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -244,6 +244,11 @@ static struct symbol *alloc_symbol(const char *name)
 	return s;
 }
 
+static uint8_t get_symbol_flags(const struct symbol *sym)
+{
+	return sym->is_gpl_only ? KSYM_FLAG_GPL_ONLY : 0;
+}
+
 /* For the hash of exported symbols */
 static void hash_add_symbol(struct symbol *sym)
 {
@@ -1865,6 +1870,9 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
 		buf_printf(buf, "KSYMTAB_%s(%s, \"%s\", \"%s\");\n",
 			   sym->is_func ? "FUNC" : "DATA", sym->name,
 			   sym->is_gpl_only ? "_gpl" : "", sym->namespace);
+
+		buf_printf(buf, "SYMBOL_FLAGS(%s, 0x%02x);\n",
+			   sym->name, get_symbol_flags(sym));
 	}
 
 	if (!modversions)
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 04/10] module loader: use kflagstab instead of *_gpl sections

[Siddharth Nayyar]

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

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/module.h   |  1 +
 kernel/module/internal.h |  1 +
 kernel/module/main.c     | 47 ++++++++++++++++++++--------------------
 3 files changed, 25 insertions(+), 24 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 3319a5269d28..9ba6ce433ac3 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -415,6 +415,7 @@ 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 c66b26184936..400d59a7f44b 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);
@@ -2607,6 +2605,7 @@ 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",
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 05/10] modpost: put all exported symbols in ksymtab section

[Siddharth Nayyar]

Since the modules loader determines whether an exported symbol is GPL
only from the kflagstab section data, modpost can put all symbols in the
regular ksymtab and stop using the *_gpl versions of the ksymtab and
kcrctab.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/export-internal.h | 21 +++++++++++----------
 scripts/mod/modpost.c           |  8 ++++----
 2 files changed, 15 insertions(+), 14 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/scripts/mod/modpost.c b/scripts/mod/modpost.c
index f5ce7aeed52d..8936db84779b 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1867,9 +1867,9 @@ 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));
@@ -1890,8 +1890,8 @@ 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);
 	}
 }
 
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 06/10] module loader: remove references of *_gpl sections

[Siddharth Nayyar]

The *_gpl section are not being used populated by modpost anymore. Hence
the module loader doesn't need to find and process these sections in
modules.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/module.h   |  3 ---
 kernel/module/internal.h |  3 ---
 kernel/module/main.c     | 40 ++++++++++++----------------------------
 3 files changed, 12 insertions(+), 34 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 9ba6ce433ac3..1a9c41318e22 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -431,9 +431,6 @@ struct module {
 	unsigned int num_kp;
 
 	/* GPL-only exported symbols. */
-	unsigned int num_gpl_syms;
-	const struct kernel_symbol *gpl_syms;
-	const u32 *gpl_crcs;
 	bool using_gplonly_symbols;
 
 #ifdef CONFIG_MODULE_SIG
diff --git a/kernel/module/internal.h b/kernel/module/internal.h
index 69b84510e097..061161cc79d9 100644
--- a/kernel/module/internal.h
+++ b/kernel/module/internal.h
@@ -53,10 +53,7 @@ extern const size_t modinfo_attrs_count;
 /* Provided by the linker */
 extern const struct kernel_symbol __start___ksymtab[];
 extern const struct kernel_symbol __stop___ksymtab[];
-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
diff --git a/kernel/module/main.c b/kernel/module/main.c
index 400d59a7f44b..4437c2a451ea 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -1464,29 +1464,18 @@ EXPORT_SYMBOL_GPL(__symbol_get);
  */
 static int verify_exported_symbols(struct module *mod)
 {
-	unsigned int i;
 	const struct kernel_symbol *s;
-	struct {
-		const struct kernel_symbol *sym;
-		unsigned int num;
-	} arr[] = {
-		{ mod->syms, mod->num_syms },
-		{ mod->gpl_syms, mod->num_gpl_syms },
-	};
-
-	for (i = 0; i < ARRAY_SIZE(arr); i++) {
-		for (s = arr[i].sym; s < arr[i].sym + arr[i].num; s++) {
-			struct find_symbol_arg fsa = {
-				.name	= kernel_symbol_name(s),
-				.gplok	= true,
-			};
-			if (find_symbol(&fsa)) {
-				pr_err("%s: exports duplicate symbol %s"
-				       " (owned by %s)\n",
-				       mod->name, kernel_symbol_name(s),
-				       module_name(fsa.owner));
-				return -ENOEXEC;
-			}
+	for (s = mod->syms; s < mod->syms + mod->num_syms; s++) {
+		struct find_symbol_arg fsa = {
+			.name	= kernel_symbol_name(s),
+			.gplok	= true,
+		};
+		if (find_symbol(&fsa)) {
+			pr_err("%s: exports duplicate symbol %s"
+				" (owned by %s)\n",
+				mod->name, kernel_symbol_name(s),
+				module_name(fsa.owner));
+			return -ENOEXEC;
 		}
 	}
 	return 0;
@@ -2601,10 +2590,6 @@ static int find_module_sections(struct module *mod, struct load_info *info)
 	mod->syms = section_objs(info, "__ksymtab",
 				 sizeof(*mod->syms), &mod->num_syms);
 	mod->crcs = section_addr(info, "__kcrctab");
-	mod->gpl_syms = section_objs(info, "__ksymtab_gpl",
-				     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
@@ -2812,8 +2797,7 @@ static int move_module(struct module *mod, struct load_info *info)
 static int check_export_symbol_versions(struct module *mod)
 {
 #ifdef CONFIG_MODVERSIONS
-	if ((mod->num_syms && !mod->crcs) ||
-	    (mod->num_gpl_syms && !mod->gpl_crcs)) {
+	if (mod->num_syms && !mod->crcs) {
 		return try_to_force_load(mod,
 					 "no versions for exported symbols");
 	}
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 07/10] linker: remove *_gpl sections from vmlinux and modules

[Siddharth Nayyar]

These sections are not used anymore and can be removed from vmlinux and
modules.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/asm-generic/vmlinux.lds.h | 18 ++----------------
 scripts/module.lds.S              |  2 --
 2 files changed, 2 insertions(+), 18 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 310e2de56211..6490b93d23b1 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -490,34 +490,20 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 									\
 	PRINTK_INDEX							\
 									\
-	/* Kernel symbol table: Normal symbols */			\
+	/* Kernel symbol table */					\
 	__ksymtab         : AT(ADDR(__ksymtab) - LOAD_OFFSET) {		\
 		__start___ksymtab = .;					\
 		KEEP(*(SORT(___ksymtab+*)))				\
 		__stop___ksymtab = .;					\
 	}								\
 									\
-	/* Kernel symbol table: GPL-only symbols */			\
-	__ksymtab_gpl     : AT(ADDR(__ksymtab_gpl) - LOAD_OFFSET) {	\
-		__start___ksymtab_gpl = .;				\
-		KEEP(*(SORT(___ksymtab_gpl+*)))				\
-		__stop___ksymtab_gpl = .;				\
-	}								\
-									\
-	/* Kernel symbol table: Normal symbols */			\
+	/* Kernel symbol CRC table */					\
 	__kcrctab         : AT(ADDR(__kcrctab) - LOAD_OFFSET) {		\
 		__start___kcrctab = .;					\
 		KEEP(*(SORT(___kcrctab+*)))				\
 		__stop___kcrctab = .;					\
 	}								\
 									\
-	/* Kernel symbol table: GPL-only symbols */			\
-	__kcrctab_gpl     : AT(ADDR(__kcrctab_gpl) - LOAD_OFFSET) {	\
-		__start___kcrctab_gpl = .;				\
-		KEEP(*(SORT(___kcrctab_gpl+*)))				\
-		__stop___kcrctab_gpl = .;				\
-	}								\
-									\
 	/* Kernel symbol flags table */					\
 	__kflagstab       : AT(ADDR(__kflagstab) - LOAD_OFFSET) {	\
 		__start___kflagstab = .;				\
diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 9a8a3b6d1569..1841a43d8771 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -20,9 +20,7 @@ SECTIONS {
 	}
 
 	__ksymtab		0 : ALIGN(8) { *(SORT(___ksymtab+*)) }
-	__ksymtab_gpl		0 : ALIGN(8) { *(SORT(___ksymtab_gpl+*)) }
 	__kcrctab		0 : ALIGN(4) { *(SORT(___kcrctab+*)) }
-	__kcrctab_gpl		0 : ALIGN(4) { *(SORT(___kcrctab_gpl+*)) }
 	__kflagstab		0 : ALIGN(1) { *(SORT(___kflagstab+*)) }
 
 	.ctors			0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) }
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 08/10] remove references to *_gpl sections in documentation

[Siddharth Nayyar]

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 Documentation/kbuild/modules.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
index d0703605bfa4..f2022fa2342f 100644
--- a/Documentation/kbuild/modules.rst
+++ b/Documentation/kbuild/modules.rst
@@ -426,11 +426,11 @@ Symbols From the Kernel (vmlinux + modules)
 Version Information Formats
 ---------------------------
 
-	Exported symbols have information stored in __ksymtab or __ksymtab_gpl
-	sections. Symbol names and namespaces are stored in __ksymtab_strings,
+	Exported symbols have information stored in the __ksymtab section.
+	Symbol names and namespaces are stored in __ksymtab_strings section,
 	using a format similar to the string table used for ELF. If
 	CONFIG_MODVERSIONS is enabled, the CRCs corresponding to exported
-	symbols will be added to the __kcrctab or __kcrctab_gpl.
+	symbols will be added to the __kcrctab section.
 
 	If CONFIG_BASIC_MODVERSIONS is enabled (default with
 	CONFIG_MODVERSIONS), imported symbols will have their symbol name and
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 09/10] modpost: add symbol import protection flag to kflagstab

[Siddharth Nayyar]

When the unused exports whitelist is provided, the symbol protection bit
is set for symbols not present in the unused exports whitelist.

The flag will be used in the following commit to prevent unsigned
modules from the using symbols other than those explicitly declared by
the such modules ahead of time.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 include/linux/module_symbol.h |  3 ++-
 scripts/mod/modpost.c         | 13 +++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/module_symbol.h b/include/linux/module_symbol.h
index 574609aced99..96fe3f4d7424 100644
--- a/include/linux/module_symbol.h
+++ b/include/linux/module_symbol.h
@@ -3,8 +3,9 @@
 #define _LINUX_MODULE_SYMBOL_H
 
 /* Kernel symbol flags bitset. */
-enum ksym_flags {
+enum symbol_flags {
 	KSYM_FLAG_GPL_ONLY	= 1 << 0,
+	KSYM_FLAG_PROTECTED	= 1 << 1,
 };
 
 /* This ignores the intensely annoying "mapping symbols" found in ELF files. */
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 8936db84779b..8d360bab50d6 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -61,6 +61,9 @@ static bool extra_warn;
 bool target_is_big_endian;
 bool host_is_big_endian;
 
+/* Are symbols protected against being used by unsigned modules? */
+static bool default_symbol_protected_status;
+
 /*
  * Cut off the warnings when there are too many. This typically occurs when
  * vmlinux is missing. ('make modules' without building vmlinux.)
@@ -225,6 +228,7 @@ struct symbol {
 	bool is_func;
 	bool is_gpl_only;	/* exported by EXPORT_SYMBOL_GPL */
 	bool used;		/* there exists a user of this symbol */
+	bool protected;		/* this symbol cannot be used by unsigned modules */
 	char name[];
 };
 
@@ -246,7 +250,8 @@ static struct symbol *alloc_symbol(const char *name)
 
 static uint8_t get_symbol_flags(const struct symbol *sym)
 {
-	return sym->is_gpl_only ? KSYM_FLAG_GPL_ONLY : 0;
+	return (sym->is_gpl_only ? KSYM_FLAG_GPL_ONLY : 0) |
+		(sym->protected ? KSYM_FLAG_PROTECTED : 0);
 }
 
 /* For the hash of exported symbols */
@@ -370,6 +375,7 @@ static struct symbol *sym_add_exported(const char *name, struct module *mod,
 	s->namespace = xstrdup(namespace);
 	list_add_tail(&s->list, &mod->exported_symbols);
 	hash_add_symbol(s);
+	s->protected = default_symbol_protected_status;
 
 	return s;
 }
@@ -1785,8 +1791,10 @@ static void handle_white_list_exports(const char *white_list)
 	while ((name = strsep(&p, "\n"))) {
 		struct symbol *sym = find_symbol(name);
 
-		if (sym)
+		if (sym) {
 			sym->used = true;
+			sym->protected = false;
+		}
 	}
 
 	free(buf);
@@ -2294,6 +2302,7 @@ int main(int argc, char **argv)
 			break;
 		case 'u':
 			unused_exports_white_list = optarg;
+			default_symbol_protected_status = true;
 			break;
 		case 'W':
 			extra_warn = true;
-- 
2.51.0.338.gd7d06c2dae-goog

[PATCH 10/10] module loader: enforce symbol import protection

[Siddharth Nayyar]

The module loader will reject unsigned modules from loading if such a
module attempts to import a symbol which has the import protection bit
set in the kflagstab entry for the symbol.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
---
 kernel/module/internal.h |  1 +
 kernel/module/main.c     | 14 ++++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/module/internal.h b/kernel/module/internal.h
index 061161cc79d9..98faaf8900aa 100644
--- a/kernel/module/internal.h
+++ b/kernel/module/internal.h
@@ -108,6 +108,7 @@ struct find_symbol_arg {
 	const u32 *crc;
 	const struct kernel_symbol *sym;
 	enum mod_license license;
+	bool is_protected;
 };
 
 /* modules using other modules */
diff --git a/kernel/module/main.c b/kernel/module/main.c
index 4437c2a451ea..ece074a6ba7b 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -380,6 +380,7 @@ static bool find_exported_symbol_in_section(const struct symsearch *syms,
 	fsa->crc = symversion(syms->crcs, sym - syms->start);
 	fsa->sym = sym;
 	fsa->license = (sym_flags & KSYM_FLAG_GPL_ONLY) ? GPL_ONLY : NOT_GPL_ONLY;
+	fsa->is_protected = sym_flags & KSYM_FLAG_PROTECTED;
 
 	return true;
 }
@@ -1273,6 +1274,11 @@ static const struct kernel_symbol *resolve_symbol(struct module *mod,
 		goto getname;
 	}
 
+	if (fsa.is_protected && !mod->sig_ok) {
+		fsa.sym = ERR_PTR(-EACCES);
+		goto getname;
+	}
+
 getname:
 	/* We must make copy under the lock if we failed to get ref. */
 	strscpy(ownername, module_name(fsa.owner), MODULE_NAME_LEN);
@@ -1550,8 +1556,12 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
 				break;
 
 			ret = PTR_ERR(ksym) ?: -ENOENT;
-			pr_warn("%s: Unknown symbol %s (err %d)\n",
-				mod->name, name, ret);
+			if (ret == -EACCES)
+				pr_warn("%s: Protected symbol %s (err %d)\n",
+					mod->name, name, ret);
+			else
+				pr_warn("%s: Unknown symbol %s (err %d)\n",
+					mod->name, name, ret);
 			break;
 
 		default:
-- 
2.51.0.338.gd7d06c2dae-goog

Re: [RFC PATCH 00/10] scalable symbol flags with __kflagstab

[Petr Pavlu]

On 8/29/25 12:54 PM, Siddharth Nayyar wrote:
> Hi everyone,
> 
> This patch series proposes a new, scalable mechanism to represent
> boolean flags for exported kernel symbols.
> 
> Problem Statement:
> 
> The core architectural issue with kernel symbol flags is our reliance on
> splitting the main symbol table, ksymtab. To handle a single boolean
> property, such as GPL-only, all exported symbols are split across two
> separate tables: __ksymtab and __ksymtab_gpl.
> 
> This design forces the module loader to perform a separate search on
> each of these tables for every symbol it needs, for vmlinux and for all
> previously loaded modules.
> 
> This approach is fundamentally not scalable. If we were to introduce a
> second flag, we would need four distinct symbol tables. For n boolean
> flags, this model requires an exponential growth to 2^n tables,
> dramatically increasing complexity.
> 
> Another consequence of this fragmentation is degraded performance. For
> example, a binary search on the symbol table of vmlinux, that would take
> only 14 comparison steps (assuming ~2^14 or 16K symbols) in a unified
> table, can require up to 26 steps when spread across two tables
> (assuming both tables have ~2^13 symbols). This performance penalty
> worsens as more flags are added.
> 
> Proposed Solution:
> 
> This series introduces a __kflagstab section to store symbol flags in a
> dedicated data structure, similar to how CRCs are handled in the
> __kcrctab.
> 
> The flags for a given symbol in __kflagstab will be located at the same
> index as the symbol's entry in __ksymtab and its CRC in __kcrctab. This
> design decouples the flags from the symbol table itself, allowing us to
> maintain a single, sorted __ksymtab. As a result, the symbol search
> remains an efficient, single lookup, regardless of the number of flags
> we add in the future.

Merging __ksymtab and __ksymtab_gpl into a single section looks ok to
me, and similarly for __kcrctab and __kcrtab_gpl. The __ksymtab_gpl
support originally comes from commit 3344ea3ad4 ("[PATCH] MODULE_LICENSE
and EXPORT_SYMBOL_GPL support") [1], where it was named __gpl_ksymtab.
The commit doesn't mention why the implementation opts for using
a separate section, but I suspect it was designed this way to reduce
memory/disk usage.

A question is whether the symbol flags should be stored in a new
__kflagstab section, instead of adding a flag member to the existing
__ksymtab. As far as I'm aware, no userspace tool (particularly kmod)
uses the __ksymtab data, so we are free to update its format.

Note that I believe that __kcrctab/__kcrtab_gpl is a separate section
because the CRC data is available only if CONFIG_MODVERSIONS=y.

Including the flags as part of __ksymtab would be obviously a simpler
schema. On the other hand, an entry in __ksymtab has in the worst case
a size of 24 bytes with an 8-byte alignment requirement. This means that
adding a flag to it would require additional 8 bytes per symbol.

> 
> The motivation for this change comes from the Android kernel, which uses
> an additional symbol flag to restrict the use of certain exported
> symbols by unsigned modules, thereby enhancing kernel security. This
> __kflagstab can be implemented as a bitmap to efficiently manage which
> symbols are available for general use versus those restricted to signed
> modules only.

I think it would be useful to explain in more detail how this protected
schema is used in practice and what problem it solves. Who is allowed to
provide these limited unsigned modules and if the concern is kernel
security, can't you enforce the use of only signed modules?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=3344ea3ad4b7c302c846a680dbaeedf96ed45c02

-- 
Thanks,
Petr