Re: [PATCH] varasm: Fix up __patchable_function_entries handling

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Dec 16, 2020 at 5:05 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Wed, Dec 02, 2020 at 05:15:21AM -0800, H.J. Lu via Gcc-patches wrote:
> > gcc/
> >
> >       PR middle-end/93195
> >       PR middle-end/93197
> >       * configure.ac (HAVE_GAS_SECTION_LINK_ORDER): New.  Define 1 if
> >       the assembler supports the section flag 'o' for specifying
> >       section with link-order.
> >       * output.h (SECTION_LINK_ORDER): New.  Defined to 0x8000000.
> >       (SECTION_MACH_DEP): Changed from 0x8000000 to 0x10000000.
> >       * targhooks.c (default_print_patchable_function_entry): Pass
> >       SECTION_LINK_ORDER to switch_to_section if the section flag 'o'
> >       works.  Pass current_function_decl to switch_to_section.
> >       * varasm.c (default_elf_asm_named_section): Use 'o' flag for
> >       SECTION_LINK_ORDER if assembler supports it.
> >       * config.in: Regenerated.
> >       * configure: Likewise.
>
> Dunno if it is an assembler bug or gcc bug, but this SECTION_LINK_ORDER
> stuff doesn't seem to work properly.
>
> If I compile:
> static inline __attribute__((__gnu_inline__)) __attribute__((__unused__)) __attribute__((patchable_function_entry(0, 0))) int foo (int x)
> {
>   return x + 1;
> }
>
> static inline __attribute__((__gnu_inline__)) __attribute__((__unused__)) __attribute__((patchable_function_entry(0, 0))) int bar (int x)
> {
>   return x + 2;
> }
>
> int
> baz (int x)
> {
>   return foo (x) + 1;
> }
>
> int
> qux (int x)
> {
>   return bar (x) + 2;
> }
> (distilled from aarch64 Linux kernel) with
> -O2 -fpatchable-function-entry=2 on aarch64 compiler configured against
> latest binutils, I get:
> ...
>         .section        __patchable_function_entries,"awo",@progbits,baz
> ...
>         .section        __patchable_function_entries
> ...
> in the assembly, but when it is assembled, one gets:
>   [ 4] __patchable_function_entries PROGBITS        0000000000000000 000060 000008 00 WAL  1   0  8
>   [ 5] .rela__patchable_function_entries RELA            0000000000000000 000280 000018 18   I 12   4  8
>   [ 6] __patchable_function_entries PROGBITS        0000000000000000 000068 000008 00      0   0  8
>   [ 7] .rela__patchable_function_entries RELA            0000000000000000 000298 000018 18   I 12   6  8
> i.e. one writable allocated section with SHF_LINK_ORDER and another
> non-allocated non-writable without link order.  In the kernel case there is
> always one entry in the WAL section and then dozens or more in the
> non-allocated one.
> The kernel then fails to link:
> WARNING: modpost: vmlinux.o (__patchable_function_entries): unexpected non-allocatable section.
> Did you forget to use "ax"/"aw" in a .S file?
> Note that for example <linux/init.h> contains
> section definitions for use in .S files.
> ld: .init.data has both ordered [`__patchable_function_entries' in init/main.o] and unordered [`.init.data' in ./drivers/firmware/efi/libstub/vsprintf.stub.o] sections
> ld: final link failed: bad value
> make: *** [Makefile:1175: vmlinux] Error 1
>
> If it is correct that the assembler requires full section flags for the
> SECTION_LINK_ORDER .section directives in every case (like it does for

gas is correct.

> comdat or for retain), then we should do something like the following
> untested change, but if it is gas bug, it should be fixed there.
>
> 2020-12-15  Jakub Jelinek  <jakub@redhat.com>
>
>         * varasm.c (default_elf_asm_named_section): Always force
>         section flags even for sections with SECTION_LINK_ORDER flag.
>
> --- gcc/varasm.c.jj     2020-12-13 17:07:53.910477664 +0100
> +++ gcc/varasm.c        2020-12-15 21:33:35.169314414 +0100
> @@ -6781,10 +6781,10 @@ default_elf_asm_named_section (const cha
>
>    /* If we have already declared this section, we can use an
>       abbreviated form to switch back to it -- unless this section is
> -     part of a COMDAT groups or with SHF_GNU_RETAIN, in which case GAS
> -     requires the full declaration every time.  */
> +     part of a COMDAT groups or with SHF_GNU_RETAIN or with SHF_LINK_ORDER,
> +     in which case GAS requires the full declaration every time.  */
>    if (!(HAVE_COMDAT_GROUP && (flags & SECTION_LINKONCE))
> -      && !(flags & SECTION_RETAIN)
> +      && !(flags & (SECTION_RETAIN | SECTION_LINK_ORDER))
>        && (flags & SECTION_DECLARED))
>      {
>        fprintf (asm_out_file, "\t.section\t%s\n", name);
>
>
>         Jakub
>

LGTM.  But I can't approve it.

Thanks.

--
H.J.