Re: [PATCH v4 3/4] zram: validate parameters in each backend's setup_params

On 08/04, Sergey Senozhatsky wrote:
> On (26/08/03 20:20), haoqin huang wrote:
> > On Thu, Jul 30, 2026 at 3:24 PM Sergey Senozhatsky
> > <[email protected]> wrote:
> > > > diff --git a/drivers/block/zram/backend_lz4hc.c b/drivers/block/zram/backend_lz4hc.c
> > > > index f6a336acfe20..5d551d165213 100644
> > > > --- a/drivers/block/zram/backend_lz4hc.c
> > > > +++ b/drivers/block/zram/backend_lz4hc.c
> > > > @@ -20,6 +20,11 @@ static int lz4hc_setup_params(struct zcomp_params *params)
> > > >  {
> > > >       if (params->level == ZCOMP_PARAM_NOT_SET)
> > > >               params->level = LZ4HC_DEFAULT_CLEVEL;
> > > > +     else if (params->level < LZ4HC_MIN_CLEVEL ||
> > > > +              params->level > LZ4HC_MAX_CLEVEL) {
> > > > +             pr_err("lz4hc: invalid compression level %d\n", params->level);
> > > > +             return -EINVAL;
> > > > +     }
> > >
> > > So...  lib/lz4/lz4hc_compress.c supports levels 1 and 2.  However,
> > > LZ4HC_MIN_CLEVEL is set to 3, but clearly the compression library
> > > supports levels lower than LZ4HC_MIN_CLEVEL.  In fact, LZ4HC_MIN_CLEVEL
> > > is never used in the lz4 code.  Maybe here we need to just hardcode
> > > "< 1" and put a comment:
> > >
> > 
> > My bad, completely missed that LZ4HC_MIN_CLEVEL is advisory and the
> > library actually accepts 1-2.  Will hardcode < 1 in v5.
> 
> No worries, that LZ4HC_MIN_CLEVEL thing is difficult to spot.
> 
> > Btw, would it make sense to fix LZ4HC_MIN_CLEVEL to 1 in the lz4
> > header as a separate cleanup?  It seems misleading as-is.
> 
> I'm afraid we cannot do that.  f2fs uses LZ4HC_MIN_CLEVEL, I assume
> compression level is stored per-inode?  So if we change LZ4HC_MIN_CLEVEL
> then newer f2fs will start accepting compression levels that older kernels
> don't support.  Cc-ed Jaegeuk and Chao just for visibility.

Yeah, since we have

 239         clevel = le16_to_cpu(ri->i_compress_flag) >>
 240                                 COMPRESS_LEVEL_OFFSET;

 255 #ifdef CONFIG_F2FS_FS_LZ4
 256 #ifdef CONFIG_F2FS_FS_LZ4HC
 257                 if (clevel &&
 258                    (clevel < LZ4HC_MIN_CLEVEL || clevel > LZ4HC_MAX_CLEVEL))
 259                         goto err_level;
 260 #else
 261                 if (clevel)
 262                         goto err_level;
 263 #endif
 264 #endif