Re: Possible bug for ZSTD kernel decompressing

[Feng Tang]

Hi Nick,

On Mon, Jan 31, 2022 at 08:31:10PM +0000, Nick Terrell wrote:
> 
> 
> > On Jan 27, 2022, at 8:53 PM, Feng Tang <feng.tang@xxxxxxxxx> wrote:
> > 
> > Hi All,
> > 
> > Recently 0Day reported a 32bit i386 kernel decompression failure for my
> > patch [1], which essentially increase the kernel data section's size
> > from 19MB to 53MB, with error message:
> > 
> > 	early console in setup code
> > 	early console in extract_kernel
> > 	input_data: 0x05077079
> > 	input_len: 0x00f8a633
> > 	output: 0x01000000
> > 	output_len: 0x045c4328
> > 	kernel_total_size: 0x05040000
> > 	needed_size: 0x05040000
> > 
> > 	Decompressing Linux... 
> > 
> > 	ZSTD-compressed data is corrupt
> > 
> > 	 -- System haltedBUG: kernel hang in boot stage
> > 
> > From debug, it is likely a problem of ZSTD decompression code, as when I
> > reverted my patch and  hacked to increase the size of kernel data
> > section by 32MB, the same error will happen.
> > 
> > Some other hints are:
> > * same i386 config with lz4 and xz algo can boot
> > * X86_64 + zstd also boots fine
> > 
> > This could be reproduced by qemu cmd:
> > 
> > qemu-system-i386 -machine pc -cpu host -enable-kvm -kernel bzImage -m 2048m -smp 4 -serial stdio  --append "earlyprintk=ttyS0,115200 console=ttyS0,115200"
> > 
> > i386 kernel config is attached, and the debug patch as below:
> > ---
> > diff --git a/init/main.c b/init/main.c
> > index 767ee2672176..873f40ddf96e 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -162,6 +162,10 @@ static size_t initargs_offs;
> > static char *execute_command;
> > static char *ramdisk_execute_command = "/init";
> > 
> > +#define DT_SIZE		8192000
> > +static unsigned long tbuf[DT_SIZE] = { 1, 2, 3, 4, };
> > +
> > /*
> >  * Used to generate warnings if static_key manipulation functions are used
> >  * before jump_label_init is called.
> > @@ -690,6 +694,11 @@ noinline void __ref rest_init(void)
> > 	struct task_struct *tsk;
> > 	int pid;
> > 
> > +	unsigned long i, j;
> > +	for (i = 0; i < DT_SIZE; i++)
> > +		j += tbuf[i];
> > +	printk("j = 0x%x\n", j);
> > +
> > 	rcu_scheduler_starting();
> > 	/*
> > 	 * We need to spawn init first so that it obtains pid 1, however
> > 
> > Please let me know if you need more info.
> > 
> > [1.] https://lore.kernel.org/lkml/1627456900-42743-1-git-send-email-feng.tang@xxxxxxxxx/ 
> 
> I've been unable to reproduce this issue using the provided patch + config based on
> Linux v5.17-rc2.
> 
> What version of Linux are you testing on? Zstd was updated in v5.16, so if you're not
> testing on v5.16 or later, can you please re-test on v5.17-rc2?
 
The original report I got is against commit 8cd7c588decf 
"mm/vmscan: throttle reclaim until some writeback completes if congested"
which is post 5.15.

I just retested and the issue can _not_ be reproduced against 5.17-rc2.
Thanks for the check and fix, and sorry for not trying latest kernel.

- Feng