Re: [PATCH] ARM: decompressor: fix BSS size calculation for LLVM ld.lld

On 05/02/2021 18:11, Ard Biesheuvel wrote:

On Fri, 5 Feb 2021 at 19:00, Nick Desaulniers [off-list ref] wrote:

quoted

On Fri, Feb 5, 2021 at 12:52 AM Ard Biesheuvel [off-list ref] wrote:

quoted

The LLVM ld.lld linker uses a different symbol type for __bss_start,
resulting in the calculation of KBSS_SZ to be thrown off. Up until now,
this has gone unnoticed as it only affects the appended DTB case, but
pending changes for ARM in the way the decompressed kernel is cleaned
from the caches has uncovered this problem.

On a ld.lld build:

  $ nm vmlinux |grep bss_
  c1c22034 D __bss_start
  c1c86e98 B __bss_stop

resulting in

$ readelf -s arch/arm/boot/compressed/vmlinux | grep bss_size

quoted

  433: c1c86e98     0 NOTYPE  GLOBAL DEFAULT  ABS _kernel_bss_size

which is obviously incorrect, and may cause the cache clean to access
unmapped memory, or cause the size calculation to wrap, resulting in no
cache clean to be performed at all.

Fix this by updating the sed regex to take D type symbols into account.

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Guillaume Tucker <redacted>
Link: https://lore.kernel.org/linux-arm-kernel/6c65bcef-d4e7-25fa-43cf-2c435bb61bb9@collabora.com/ (local)
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Thanks for debugging+fixing this, and Guillaume for the report.  It's
nice to see a fix so early; thinking back to last year before KernelCI
integration, we probably would have only noticed when CrOS went to
upgrade their rk3288 platform devices.

Some other tags that might be nice to apply:

Cc: stable@kernel.org
Fixes: 429f7a062e3b ("ARM: decompressor: fix BSS size calculation")
Reported-by: Guillaume Tucker <redacted>
Reported-by: "kernelci.org bot" <redacted>

Thanks. I'll add these tags and drop this patch into the patch system

Thanks!  You may even add:

  Tested-by: "kernelci.org bot" [off-list ref]

See some results here, showing it's all booting fine:

  https://staging.kernelci.org/test/job/ardb/branch/for-kernelci/kernel/v5.11-rc6-146-g923ca344043a/plan/baseline/

with this revision from your for-kernelci branch:

  https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?id=923ca344043a19067c21f46bd8649f35f61ce920

Best wishes,
Guillaume

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel