objtool: Fix C jump table annotations for Clang

[ Upstream commit 73cfc53cc3b6380eccf013049574485f64cb83ca ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 75a83394824c47a169ab2480b94ebe42396a9ad8..b15911e201bf95647dc89df8a61cfaa3144f68ca 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 3e72c9f9b7f3753286262cef9e1bc6ececac3647..1691aa6e6ce32d627bcaace436a0893b85cd3507 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2589,13 +2589,14 @@ 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 86d4af9c5aa9dc83cd543a140ab1979979a1d6fa..89ee12b1a13849ef0f3a3c6a28f26ce32f484819 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;