Andreas Dilger <adilger@xxxxxxxxx> 于2023年8月2日周三 14:07写道：
>
> Not all of these cases are actual bugs. The ext4_rec_len_from_disk()
> function is only different for rec_len >= 2^16, so if it is comparing
> rec_len against "12" or "sizeof(struct ...)" then the inequality will
> be correct regardless of how it is decoded.
>
> That said, it makes sense to use ext4_rec_len_from_disk() to access
> rec_len consistently throughout the code, since that avoids potential
> bugs in the future. We know the code will eventually will be copied
> some place where rec_len >= 2^16 is actually important, and we may as
> well avoid that bug before it happens.
>
>
> One thing this discussion *does* expose is that ext4_rec_len_from_disk()
> is hard-coded at compile time to differentiate between PAGE_SIZE > 64k
> and PAGE_SIZE = 4K, because it was never possible to have blocksize >
> PAGE_SIZE, so only ARM/PPC ever had filesystems with blocksize=64KiB
> (and the Fujitsu Fugaku SPARC system with blocksize=256KiB).
>
> However, with the recent advent of the VM and IO layers allowing
> blocksize > PAGE_SIZE this function will need to be changed to allow
> the same on x86 PAGE_SIZE=4KiB systems. Instead of checking
>
> #if PAGE_SIZE >= 65536
>
> it should handle this based on the filesystem blocksize at runtime:
>
> static inline
> unsigned int ext4_rec_len_from_disk(__le16 dlen, unsigned blocksize)
> {
> unsigned len = le16_to_cpu(dlen);
>
> if (blocksize < 65536)
> return len;
>
> if (len == EXT4_MAX_REC_LEN || len == 0)
> return blocksize;
>
> return (len & 65532) | ((len & 3) << 16);
> }
>
> Strictly speaking, ((len & 65532) | ((len & 3) << 16) should equal "len"
> for any filesystem with blocksize < 65536, but IMHO it is more clear if
> the code is written this way.
>
> Similarly, the encoding needs to be changed to handle large records at
> runtime for when we eventually allow ext4 with blocksize > PAGE_SIZE.
>
> static inline __le16 ext4_rec_len_to_disk(unsigned len, unsigned blocksize)
> {
> BUG_ON(len > blocksize);
> BUG_ON(blocksize > (1 << 18));
> BUG_ON(len & 3);
>
> if (len < 65536) /* always true for blocksize < 65536 */
> return cpu_to_le16(len);
>
> if (len == blocksize) {
> if (blocksize == 65536)
> return cpu_to_le16(EXT4_MAX_REC_LEN);
>
> return cpu_to_le16(0);
> }
>
> return cpu_to_le16((len & 65532) | ((len >> 16) & 3));
> }
>
Hmm, at least it sounds reasonable to me based on my limited
knowledge. However, I am not sure whether you want me to incorporate
these changes into this particular commit or another patch within this
submission.
By default, I will simply leave it for further discussion. Please let
me know if you have any ideas.
Cheers,
Shida
>
> Cheers, Andreas
>
>
>
>
>
