llext: add support for SLID-based linking

This commit introduces support for an alternate linking method in the
LLEXT subsystem, called "SLID" (short for Symbol Link Identifier),
enabled by the CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID Kconfig option.

SLID-based linking uses a unique identifier (integer) to identify
exported symbols, instead of using the symbol name as done currently.
This approach provides several benefits:
 * linking is faster because the comparison operation to determine
   whether we found the correct symbol in the export table is now an
   integer compare, instead of a string compare
 * binary size is reduced as symbol names can be dropped from the binary
 * confidentiality is improved as a side-effect, as symbol names are no
   longer present in the binary

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>

tests/subsys/llext/simple/src/test_llext_simple.c

@@ -24,6 +24,20 @@ LOG_MODULE_REGISTER(test_llext_simple);
 #define LLEXT_CONST const
 #endif
 
+#ifdef CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID
+#define LLEXT_FIND_BUILTIN_SYM(symbol_name) llext_find_sym(NULL, symbol_name ## _SLID)
+
+#ifdef CONFIG_64BIT
+#define printk_SLID ((const char *)0x87B3105268827052ull)
+#define z_impl_ext_syscall_fail_SLID ((const char *)0xD58BC0E7C64CD965ull)
+#else
+#define printk_SLID ((const char *)0x87B31052ull)
+#define z_impl_ext_syscall_fail_SLID ((const char *)0xD58BC0E7ull)
+#endif
+#else
+#define LLEXT_FIND_BUILTIN_SYM(symbol_name) llext_find_sym(NULL, # symbol_name)
+#endif
+
 struct llext_test {
 	const char *name;
 	bool try_userspace;
@@ -271,7 +285,7 @@ ZTEST(llext, test_pre_located)
  */
 ZTEST(llext, test_printk_exported)
 {
-	const void * const printk_fn = llext_find_sym(NULL, "printk");
+	const void * const printk_fn = LLEXT_FIND_BUILTIN_SYM(printk);
 
 	zassert_equal(printk_fn, printk, "printk should be an exported symbol");
 }
@@ -282,8 +296,9 @@ ZTEST(llext, test_printk_exported)
  */
 ZTEST(llext, test_ext_syscall_fail)
 {
-	const void * const esf_fn = llext_find_sym(NULL,
-						   "z_impl_ext_syscall_fail");
+	const void * const esf_fn = LLEXT_FIND_BUILTIN_SYM(z_impl_ext_syscall_fail);
+
+	zassert_not_null(esf_fn, "est_fn should not be NULL");
 
 	zassert_is_null(*(uintptr_t **)esf_fn, NULL,
 			"ext_syscall_fail should be NULL");

tests/subsys/llext/simple/testcase.yaml

@@ -80,3 +80,71 @@ tests:
       - CONFIG_USERSPACE=y
       - CONFIG_MODULES=y
       - CONFIG_LLEXT_TEST_HELLO=m
+  llext.simple.readonly_slid_linking:
+    arch_exclude: xtensa # for now
+    filter: not CONFIG_MPU and not CONFIG_MMU and not CONFIG_SOC_SERIES_S32ZE
+    extra_configs:
+      - arch:arm:CONFIG_ARM_MPU=n
+      - CONFIG_LLEXT_STORAGE_WRITABLE=n
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y
+  llext.simple.readonly_mpu_slid_linking:
+    min_ram: 128
+    arch_exclude: xtensa # for now
+    filter: CONFIG_ARCH_HAS_USERSPACE
+    extra_configs:
+      - CONFIG_USERSPACE=y
+      - CONFIG_LLEXT_STORAGE_WRITABLE=n
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y
+  llext.simple.writable_slid_linking:
+    filter: not CONFIG_MPU and not CONFIG_MMU and not CONFIG_SOC_SERIES_S32ZE
+    extra_configs:
+      - arch:arm:CONFIG_ARM_MPU=n
+      - CONFIG_LLEXT_STORAGE_WRITABLE=y
+  llext.simple.modules_enabled_writable_slid_linking:
+    filter: not CONFIG_MPU and not CONFIG_MMU
+    platform_key:
+      - simulation
+      - arch
+    platform_exclude:
+      - qemu_cortex_a9 # MMU
+    extra_configs:
+      - arch:arm:CONFIG_ARM_MPU=n
+      - CONFIG_MODULES=y
+      - CONFIG_LLEXT_STORAGE_WRITABLE=y
+      - CONFIG_LLEXT_TEST_HELLO=m
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y
+  llext.simple.modules_enabled_writable_relocatable_slid_linking:
+    arch_exclude: arm arm64
+    filter: not CONFIG_MPU and not CONFIG_MMU
+    integration_platforms:
+      - qemu_xtensa
+    extra_configs:
+      - CONFIG_MODULES=y
+      - CONFIG_LLEXT_STORAGE_WRITABLE=y
+      - CONFIG_LLEXT_TYPE_ELF_RELOCATABLE=y
+      - CONFIG_LLEXT_TEST_HELLO=m
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y
+  llext.simple.modules_enabled_readonly_slid_linking:
+    filter: not CONFIG_MPU and not CONFIG_MMU
+    arch_exclude: xtensa # for now
+    platform_key:
+      - simulation
+      - arch
+    platform_exclude:
+      - qemu_cortex_a9 # MMU
+    extra_configs:
+      - arch:arm:CONFIG_ARM_MPU=n
+      - CONFIG_MODULES=y
+      - CONFIG_LLEXT_TEST_HELLO=m
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y
+  llext.simple.modules_enabled_readonly_mpu_slid_linking:
+    filter: CONFIG_ARCH_HAS_USERSPACE
+    arch_exclude: xtensa # for now
+    platform_key:
+      - simulation
+      - arch
+    extra_configs:
+      - CONFIG_USERSPACE=y
+      - CONFIG_MODULES=y
+      - CONFIG_LLEXT_TEST_HELLO=m
+      - CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID=y