Re: [PATCH] compiler.h: Specify correct attribute for .rodata..c_jump_table

[Tiezhu Yang]

On 02/14/2025 02:54 AM, Josh Poimboeuf wrote:
> On Thu, Feb 13, 2025 at 08:38:22PM +0800, Tiezhu Yang wrote:
> > Based on the above analysis, in order to avoid changing the behavior of
> > GNU assembler and silence the GNU assembler warning, is it possible to
> > rename ".rodata..c_jump_table" to ".c_jump_table"?
>
> Yeah, though for the final vmlinux link we'd still need it to end up in
> .rodata so it remains read-only at runtime.
>
> So we'd basically just have to add it to the RO_DATA() macro in
> include/asm-generic/vmlinux.lds.h.
>
> Does it still work if we call it ..rodata.c_jump_table or so?  Then we
> could just add "*(..rodata.*) to RO_DATA().  That would be somewhat less
> surprising, and would also create a standard way for other code to do
> the same thing by prefixing with "..rodata.".

"..rodata.c_jump_table" works well with GCC and Clang,
there is no assembler warning for GCC and the section
name is normal for Clang.

I have a draft diff, I will do more test and send it
as a formal patch once the other patches are landed
because this patch depends on them.

The code looks something like this:

-- >8 --
diff --git a/include/asm-generic/vmlinux.lds.h 
b/include/asm-generic/vmlinux.lds.h
index 02a4adb4a999..0af80d6627fa 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -457,7 +457,7 @@ defined(CONFIG_AUTOFDO_CLANG) || 
defined(CONFIG_PROPELLER_CLANG)
        . = ALIGN((align));                                             \
        .rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {           \
                __start_rodata = .;                                     \
-               *(.rodata) *(.rodata.*)                                 \
+               *(.rodata) *(.rodata.*) *(..rodata.*)                   \
                SCHED_DATA                                              \
                RO_AFTER_INIT_DATA      /* Read only after init */      \
                . = ALIGN(8);                                           \
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 200fd3c5bc70..9b56788bf3f1 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -110,7 +110,8 @@ 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("..rodata.c_jump_table")
+#endif
 #else /* !CONFIG_OBJTOOL */
 #define __annotate_jump_table
 #endif /* CONFIG_OBJTOOL */
diff --git a/tools/objtool/arch/loongarch/special.c 
b/tools/objtool/arch/loongarch/special.c
index 27c6473608f3..d3e10ad3a9bf 100644
--- a/tools/objtool/arch/loongarch/special.c
+++ b/tools/objtool/arch/loongarch/special.c
@@ -131,7 +131,7 @@ static struct reloc 
*find_reloc_of_rodata_c_jump_table(struct section *sec,
                if (reloc_offset(reloc) > offset)
                        break;

-               if (!strncmp(reloc->sym->sec->name, 
".rodata..c_jump_table", 21)) {
+               if (!strncmp(reloc->sym->sec->name, 
"..rodata.c_jump_table", 21)) {
                        *table_size = 0;
                        return reloc;
                }
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 7174dcb7496e..a8b67b4c5726 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2482,12 +2482,13 @@ 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
+        * - ..rodata.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) &&
+               if ((!strncmp(sec->name, ".rodata", 7) ||
+                   !strncmp(sec->name, "..rodata", 8)) &&
                    !strstr(sec->name, ".str1.")) {
                        sec->rodata = true;
                        found = true;
diff --git a/tools/objtool/include/objtool/special.h 
b/tools/objtool/include/objtool/special.h
index e7ee7ffccefd..34acf4ae5fab 100644
--- 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 "..rodata.c_jump_table"

 struct special_alt {
        struct list_head list;

Thanks,
Tiezhu