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objtool: Fix C jump table annotations for Clang

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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

  c5b1184dec ("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: c5b1184dec ("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>
This commit is contained in:
Ard Biesheuvel 2025-02-21 14:57:07 +01:00 • committed by Josh Poimboeuf
parent 68f3ea7ee1
commit 73cfc53cc3
3 changed files with 6 additions and 5 deletions
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2
include/linux/compiler.h
View file

@ -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 */

7
tools/objtool/check.c
View file

@ -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;
}

2
tools/objtool/include/objtool/special.h
View file

@ -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;