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

  c5b1184d ("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: c5b1184d ("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>