On Thu, Jul 02, 2020 at 08:54:53AM -0700, Kees Cook wrote:
> On Thu, Jul 02, 2020 at 01:54:00AM -0700, Danny Lin wrote:
> > A recent LLVM 11 commit [1] made LLD stop implicitly coalescing some
> > temporary LLVM sections, namely .{data,bss}..compoundliteral.XXX:
> >
> > [30] .data..compoundli PROGBITS ffffffff9ac9a000 19e9a000
> > 000000000000cea0 0000000000000000 WA 0 0 32
> > [31] .rela.data..compo RELA 0000000000000000 40965440
> > 0000000000001d88 0000000000000018 I 2238 30 8
> > [32] .data..compoundli PROGBITS ffffffff9aca6ea0 19ea6ea0
> > 00000000000033c0 0000000000000000 WA 0 0 32
> > [33] .rela.data..compo RELA 0000000000000000 409671c8
> > 0000000000000948 0000000000000018 I 2238 32 8
> > [...]
> > [2213] .bss..compoundlit NOBITS ffffffffa3000000 1d85c000
> > 00000000000000a0 0000000000000000 WA 0 0 32
> > [2214] .bss..compoundlit NOBITS ffffffffa30000a0 1d85c000
> > 0000000000000040 0000000000000000 WA 0 0 32
> > [...]
> >
> > While these extra sections don't typically cause any breakage, they do
> > inflate the vmlinux size due to the overhead of storing metadata for
> > thousands of extra sections.
> >
> > It's also worth noting that for some reason, some downstream Android
> > kernels can't boot at all if these sections aren't coalesced.
> >
> > This issue isn't limited to any specific architecture; it affects arm64
> > and x86 if CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is forced on.
It might be worth noting that this happens explicitly because of
-fdata-sections, which is currently only used with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION but there are other features such
as LTO that will enable this and make this relevant in the future.
https://android-review.googlesource.com/c/kernel/common/+/1329278/6#message-81b191e92ef4e98e017fa9ded5ef63ef6e60db3a
It is also worth noting that those commits add .bss..L* and .data..L*
and rodata variants. Do you know if those are relevant here?
> > Example on x86 allyesconfig:
> > Before: 2241 sections, 1170972 KiB
> > After: 56 sections, 1171169 KiB
Am I reading this right that coalescing those sections increases the
image size? Kind of interesting.
> > [1] https://github.com/llvm/llvm-project/commit/9e33c096476ab5e02ab1c8442cc3cb4e32e29f17
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/958
> > Cc: stable@xxxxxxxxxxxxxxx # v4.4+
> > Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
> > Signed-off-by: Danny Lin <danny@xxxxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > ---
> > include/asm-generic/vmlinux.lds.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index db600ef218d7..18968cba87c7 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -94,10 +94,10 @@
> > */
> > #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
> > #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
> > -#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
> > +#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX* .data..compoundliteral*
I am fairly certain this will fix a PowerPC warning that we had
recently so good!
https://lore.kernel.org/lkml/202006180904.TVUXCf6H%25lkp@xxxxxxxxx/
Unfortunately, I forgot to reply to that thread...
> > #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
> > #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
> > -#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
> > +#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*
>
> Are there .data.. and .bss.. sections we do NOT want to collect? i.e.
> why not include .data..* and .bss..* ?
At one point Android was doing that for modules but stopped:
https://android-review.googlesource.com/c/kernel/common/+/1266787
I wonder if that is a problem for the main kernel image.
Cheers,
Nathan
