Re: Possible bug for ZSTD kernel decompressing

[Nick Terrell <terrelln@fb.com>]

> On Jan 27, 2022, at 8:53 PM, Feng Tang <feng.tang@intel.com> 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@intel.com/ 

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?

Best,
Nick Terrell

> Thanks,
> Feng
> <i386_decompress_fail.config>