linux-kernel - Re: [PATCH] btrfs: Accept and ignore compression level for lzo

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Message-ID: <663c2f5b-3bb1-4a40-b962-11c6d3a7f806@gmx.com>
Date: Mon, 25 Aug 2025 18:33:11 +0930
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Daniel Vacek <neelx@...e.com>, Calvin Owens <calvin@...nvd.org>
Cc: Sun YangKai <sunk67188@...il.com>, clm@...com, dsterba@...e.com,
 josef@...icpanda.com, linux-btrfs@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: Accept and ignore compression level for lzo



在 2025/8/25 18:21, Daniel Vacek 写道:
> On Sun, 24 Aug 2025 at 17:58, Calvin Owens <calvin@...nvd.org> wrote:
>> From: Calvin Owens <calvin@...nvd.org>
>> Subject: [PATCH v3] btrfs: Accept and ignore compression level for lzo
>>
>> The compression level is meaningless for lzo, but before commit
>> 3f093ccb95f30 ("btrfs: harden parsing of compression mount options"),
>> it was silently ignored if passed.
>>
>> After that commit, passing a level with lzo fails to mount:
>>
>>      BTRFS error: unrecognized compression value lzo:1
>>
>> It seems reasonable for users to expect that lzo would permit a numeric
>> level option, as all the other algos do, even though the kernel's
>> implementation of LZO currently only supports a single level. Because it
>> has always worked to pass a level, it seems likely to me that users in
>> the real world are relying on doing so.
>>
>> This patch restores the old behavior, giving "lzo:N" the same semantics
>> as all of the other compression algos.
>>
>> To be clear, silly variants like "lzo:one", "lzo:the_first_option", or
>> "lzo:armageddon" also used to work. This isn't meant to suggest that
>> any possible mis-interpretation of mount options that once worked must
>> continue to work forever. This is an exceptional case where it makes
>> sense to preserve compatibility, both because the mis-interpretation is
>> reasonable, and because nothing tangible is sacrificed.
>>
>> Fixes: 3f093ccb95f30 ("btrfs: harden parsing of compression mount options")
>> Signed-off-by: Calvin Owens <calvin@...nvd.org>
>> ---
>>   fs/btrfs/super.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> v3 looks good to me. The original hardening was meant to gate complete
> nonsense like "compress=lzoutput", etc...
> 
> Reviewed-by: Daniel Vacek <neelx@...e.com>

Now merged and pushed to for-next branch with the latest reviewed-by tags.

Thanks,
Qu
> 
> Thank you.
> 
>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>> index a262b494a89f..18eb00b3639b 100644
>> --- a/fs/btrfs/super.c
>> +++ b/fs/btrfs/super.c
>> @@ -299,9 +299,12 @@ static int btrfs_parse_compress(struct btrfs_fs_context *ctx,
>>                  btrfs_set_opt(ctx->mount_opt, COMPRESS);
>>                  btrfs_clear_opt(ctx->mount_opt, NODATACOW);
>>                  btrfs_clear_opt(ctx->mount_opt, NODATASUM);
>> -       } else if (btrfs_match_compress_type(string, "lzo", false)) {
>> +       } else if (btrfs_match_compress_type(string, "lzo", true)) {
>>                  ctx->compress_type = BTRFS_COMPRESS_LZO;
>> -               ctx->compress_level = 0;
>> +               ctx->compress_level = btrfs_compress_str2level(BTRFS_COMPRESS_LZO,
>> +                                                              string + 3);
>> +               if (string[3] == ':' && string[4])
>> +                       btrfs_warn(NULL, "Compression level ignored for LZO");
>>                  btrfs_set_opt(ctx->mount_opt, COMPRESS);
>>                  btrfs_clear_opt(ctx->mount_opt, NODATACOW);
>>                  btrfs_clear_opt(ctx->mount_opt, NODATASUM);
>> --
>> 2.49.1
>>