linux-kernel - Re: Linux 6.2-rc2

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Date:   Tue, 3 Jan 2023 11:58:48 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: Linux 6.2-rc2

On Tue, 3 Jan 2023 at 03:13, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, Jan 2, 2023 at 5:45 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >
> > ... and reverting commit 99cb0d917ff indeed fixes the problem.
>
> Hmm. My gut feel is that this just exposes some bug in binutils.
>
> That said, maybe that commit should not have added its own /DISCARDS/
> thing, and instead just added that "*(.note.GNU-stack)" to the general
> /DISCARDS/ thing that is defined by the
>
>   #define DISCARDS  ..
>
> a little bit later, so that we only end up with one single DISCARD
> list. Something like this (broken patch on purpose):
>
>   --- a/include/asm-generic/vmlinux.lds.h
>   +++ b/include/asm-generic/vmlinux.lds.h
>   @@ -897,5 +897,4 @@
>     */
>    #define NOTES                                        \
>   -     /DISCARD/ : { *(.note.GNU-stack) }              \
>         .notes : AT(ADDR(.notes) - LOAD_OFFSET) {       \
>                 BOUNDED_SECTION_BY(.note.*, _notes)     \
>   @@ -1016,4 +1015,5 @@
>    #define DISCARDS                                     \
>         /DISCARD/ : {                                   \
>   +     *(.note.GNU-stack)                              \
>         EXIT_DISCARDS                                   \
>         EXIT_CALL                                       \
>
> But maybe that DISCARDS macrop ends up being used too late?
>

Masahiro's v1 did something like this, and it caused an issue on
RISC-V, which is why we ended up with this approach instead.

> It really shouldn't matter, but here we are, with a build problem with
> some random old binutils on an odd platform..
>

AIUI, the way ld.bfd used to combine output sections may also affect
the /DISCARD/ pseudo-section, and so introducing it much earlier
results in these discards to be interpreted in a different order.

The purpose of this change is to prevent .note.GNU-stack from deciding
the section type of the .notes output section, and so keeping it in
its own section should be sufficient. E.g.,

--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -896,7 +896,7 @@
  * Otherwise, the type of .notes section would become PROGBITS
instead of NOTES.
  */
 #define NOTES                                                          \
-       /DISCARD/ : { *(.note.GNU-stack) }                              \
+       .note.GNU-stack : { *(.note.GNU-stack) }                        \
        .notes : AT(ADDR(.notes) - LOAD_OFFSET) {                       \
                BOUNDED_SECTION_BY(.note.*, _notes)                     \
        } NOTES_HEADERS                                                 \

The .note.GNU-stack has zero size, so the result should be the same.