Re: [PATCH 0/2] riscv: improve __ex_table section handling

Palmer Dabbelt
2021-08-26T06:47:38Z

On Sun, 08 Aug 2021 10:25:09 PDT (-0700), jszhang3@mail.ustc.edu.cn wrote:
> From: Jisheng Zhang <jszhang@kernel.org>
>
> Enable BUILDTIME_TABLE_SORT to sort the exception table at build time
> then move exception table to RO_DATA segment.
>
> Jisheng Zhang (2):
> riscv: Enable BUILDTIME_TABLE_SORT
> riscv: Move EXCEPTION_TABLE to RO_DATA segment
>
> arch/riscv/Kconfig | 1 +
> arch/riscv/kernel/vmlinux-xip.lds.S | 1 -
> arch/riscv/kernel/vmlinux.lds.S | 4 ++--
> scripts/sorttable.c | 1 +
> 4 files changed, 4 insertions(+), 3 deletions(-)

This seems reasonable, but it's failing for some configurations (at
least tinyconfig) saying there is no __ex_table. I'm not entirely sure
how that comes about, as we've got them for futexes and uaccess.

Maybe the right thing to do here is to fix scripts/sorttable.c so it can
handle files with nothing to sort? I think it's just as simple as a
successful early out like this

diff --git a/scripts/sorttable.h b/scripts/sorttable.h
index a2baa2fefb13..207ddeddb506 100644
--- a/scripts/sorttable.h
+++ b/scripts/sorttable.h
@@ -294,8 +294,9 @@ static int do_sort(Elf_Ehdr *ehdr,
goto out;
}
#endif
+ /* If there is no __ex_table section there is no work do to. */
if (!extab_sec) {
- fprintf(stderr, "no __ex_table in file: %s\n", fname);
+ rc = 0;
goto out;
}

I'm not entirely sure though -- my logic is essentially just "there's no
__ex_table, so there's nothing to sort, so just don't try".

All the configurations I can actually boot have an __ex_table, so I'm
not sure how to test that.