objtool: Fix C jump table annotations for Clang

A C jump table (such as the one used by the BPF interpreter) is a const
global array of absolute code addresses, and this means that the actual
values in the table may not be known until the kernel is booted (e.g.,
when using KASLR or when the kernel VA space is sized dynamically).

When using PIE codegen, the compiler will default to placing such const
global objects in .data.rel.ro (which is annotated as writable), rather
than .rodata (which is annotated as read-only). As C jump tables are
explicitly emitted into .rodata, this used to result in warnings for
LoongArch builds (which uses PIE codegen for the entire kernel) like

  Warning: setting incorrect section attributes for .rodata..c_jump_table

due to the fact that the explicitly specified .rodata section inherited
the read-write annotation that the compiler uses for such objects when
using PIE codegen.

This warning was suppressed by explicitly adding the read-only
annotation to the __attribute__((section(""))) string, by commit

  c5b1184decc8 ("compiler.h: specify correct attribute for .rodata..c_jump_table")

Unfortunately, this hack does not work on Clang's integrated assembler,
which happily interprets the appended section type and permission
specifiers as part of the section name, which therefore no longer
matches the hard-coded pattern '.rodata..c_jump_table' that objtool
expects, causing it to emit a warning

  kernel/bpf/core.o: warning: objtool: ___bpf_prog_run+0x20: sibling call from callable instruction with modified stack frame

Work around this, by emitting C jump tables into .data.rel.ro instead,
which is treated as .rodata by the linker script for all builds, not
just PIE based ones.

Fixes: c5b1184decc8 ("compiler.h: specify correct attribute for .rodata..c_jump_table")
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn> # on LoongArch
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250221135704.431269-6-ardb+git@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index b087de2f3e94b5bef06ceb805713cda8dc7c9894..0c25f3e429bba85f71e188943d8069db6b38f98e 100644 (file)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -110,7 +110,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 /* Unreachable code */
 #ifdef CONFIG_OBJTOOL
 /* Annotate a C jump table to allow objtool to follow the code flow */
-#define __annotate_jump_table __section(".rodata..c_jump_table,\"a\",@progbits #")
+#define __annotate_jump_table __section(".data.rel.ro.c_jump_table")
 #else /* !CONFIG_OBJTOOL */
 #define __annotate_jump_table
 #endif /* CONFIG_OBJTOOL */

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 497cb8dfb3eb3b164fd5d9413f1c364764acac30..1b5a1b3ea7a9f8bddce29aca951b026f16eed62d 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2471,13 +2471,14 @@ static void mark_rodata(struct objtool_file *file)
         *
         * - .rodata: can contain GCC switch tables
         * - .rodata.<func>: same, if -fdata-sections is being used
-        * - .rodata..c_jump_table: contains C annotated jump tables
+        * - .data.rel.ro.c_jump_table: contains C annotated jump tables
         *
         * .rodata.str1.* sections are ignored; they don't contain jump tables.
         */
        for_each_sec(file, sec) {
-               if (!strncmp(sec->name, ".rodata", 7) &&
-                   !strstr(sec->name, ".str1.")) {
+               if ((!strncmp(sec->name, ".rodata", 7) &&
+                    !strstr(sec->name, ".str1.")) ||
+                   !strncmp(sec->name, ".data.rel.ro", 12)) {
                        sec->rodata = true;
                        found = true;
                }

diff --git a/tools/objtool/include/objtool/special.h b/tools/objtool/include/objtool/special.h
index e7ee7ffccefd4d4ef520767b1c3a4f86f042780b..e049679bb17b2668f53e27646acad43ec79c40f7 100644 (file)
--- a/tools/objtool/include/objtool/special.h
+++ b/tools/objtool/include/objtool/special.h
@@ -10,7 +10,7 @@
 #include <objtool/check.h>
 #include <objtool/elf.h>
 
-#define C_JUMP_TABLE_SECTION ".rodata..c_jump_table"
+#define C_JUMP_TABLE_SECTION ".data.rel.ro.c_jump_table"
 
 struct special_alt {
        struct list_head list;