Re: [PATCH 3/5] kbuild: rename cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}

[Eugeniu Rosca <erosca@de.adit-jv.com>]

Hello Yamada-san,

On Fri, Jun 23, 2023 at 04:45:44PM +0200, Eugeniu Rosca wrote:
> Hello Yamada-san,
> Hello Nicolas,
> Cc: SzuWei Lin (committer of the patch in AOSP [1])
> Cc: Kbuild
> 
> On Mon, Jan 10, 2022 at 12:33:15PM +0100, Nicolas Schier wrote:
> > On Mon, Jan 10, 2022 at 03:15:27AM +0900, Masahiro Yamada wrote:
> > > GZIP-compressed files end with 4 byte data that represents the size
> > > of the original input. The decompressors (the self-extracting kernel)
> > > exploit it to know the vmlinux size beforehand. To mimic the GZIP's
> > > trailer, Kbuild provides cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}.
> > > Unfortunately these macros are used everywhere despite the appended
> > > size data is only useful for the decompressors.
> > > 
> > > There is no guarantee that such hand-crafted trailers are safely ignored.
> > > In fact, the kernel refuses compressed initramdisks with the garbage
> > > data. That is why usr/Makefile overrides size_append to make it no-op.
> > > 
> > > To limit the use of such broken compressed files, this commit renames
> > > the existing macros as follows:
> > > 
> > >   cmd_bzip2   --> cmd_bzip2_with_size
> > >   cmd_lzma    --> cmd_lzma_with_size
> > >   cmd_lzo     --> cmd_lzo_with_size
> > >   cmd_lz4     --> cmd_lz4_with_size
> > >   cmd_xzkern  --> cmd_xzkern_with_size
> > >   cmd_zstd22  --> cmd_zstd22_with_size
> > > 
> > > To keep the decompressors working, I updated the following Makefiles
> > > accordingly:
> > > 
> > >   arch/arm/boot/compressed/Makefile
> > >   arch/h8300/boot/compressed/Makefile
> > >   arch/mips/boot/compressed/Makefile
> > >   arch/parisc/boot/compressed/Makefile
> > >   arch/s390/boot/compressed/Makefile
> > >   arch/sh/boot/compressed/Makefile
> > >   arch/x86/boot/compressed/Makefile
> > > 
> > > I reused the current macro names for the normal usecases; they produce
> > > the compressed data in the proper format.
> > > 
> > > I did not touch the following:
> > > 
> > >   arch/arc/boot/Makefile
> > >   arch/arm64/boot/Makefile
> > >   arch/csky/boot/Makefile
> > >   arch/mips/boot/Makefile
> > >   arch/riscv/boot/Makefile
> > >   arch/sh/boot/Makefile
> > >   kernel/Makefile
> > > 
> > > This means those Makefiles will stop appending the size data.
> > > 
> > > I dropped the 'override size_append' hack from usr/Makefile.
> > > 
> > > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > > ---
> > 
> > Reviewed-by: Nicolas Schier <n.schier@avm.de>
> 
> If you don't mind, I would like to report another instance of
> "/bin/sh: Argument list too long" while building some out-of-tree *ko
> in a number of downstream v5.15.78+ kernels containing [1].
> 
> For some time now, we've been living with ugly hacks to overcome it.
> 
> Fortunately, recent git bisecting efforts apparently reveal that
> current v5.17-rc1 commit (and its backports in downstream) look to
> act as the culprit (confirmed on several host machines). So, I
> started to have some hopes of a long-term solution and hence
> sharing the findings as a first step.
> 
> I am not entirely clear how to properly trace this behavior, since no
> amount of "make V=1/V=2" uncovers more details. Purely by accident, I
> looked into the top/htop output (while running the repro) and
> noticed several processes doing:
> 
> /bin/sh -c dec_size=0; for F in <humongous list of filenames>; do \
>   fsize=$(sh /abs/path/to/scripts/file-size.sh $F); \
>   dec_size=$(expr $dec_size + $fsize); done; printf "%08x\n" $dec_size \
>   | sed 's/\(..\)/\1 /g' | { read ch0 ch1 ch2 ch3; for ch in \
>   $ch3 $ch2 $ch1 $ch0; do printf '%s%03o' '\\' $((0x$ch)); done; }
> 
> As it was the case in the recent report [2], the above command seems
> to require/assume generous amount of space for the shell arguments.
> 
> I still haven't compared the exact traces before and after this commit,
> to quantify by how much the shell argument list is increased (TODO).
> 
> Another aspect is that current commit seems to introduce the
> regression in a multi-threaded make only. The issue is apparently
> masked by 'make -j1' (TBC), which adds another level of complexity.
> 
> Unfortunately, the build use-case is highly tailored to downstream
> and is not repeatable against vanilla out of the box.
> 
> I will continue to increase my understanding behind what's happening.
> In case there are already any suggestions, would appreciate those.

JFYI, we've got confirmation from Qualcomm Customer Support interface
that reverting [1] heals the issue on QC end as well. However, it looks
like none of us has clear understanding how to properly
troubleshoot/trace/compare the behavior before and after the commit.

I would happily follow any suggestions.

> [1] https://android.googlesource.com/kernel/common/+/bc6d3d83539512
>     ("UPSTREAM: kbuild: rename cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}")
> 
> [2] https://lore.kernel.org/linux-kbuild/20230616194505.GA27753@lxhi-065/

-- 
Best regards,
Eugeniu Rosca