[bug report] erofs: enable error reporting for z_erofs_fixup_insize()

[Dan Carpenter]

Hello Gao Xiang,

Commit 30e13e41a0eb ("erofs: enable error reporting for
z_erofs_fixup_insize()") from Nov 27, 2025 (linux-next), leads to the
following Smatch static checker warning:

fs/erofs/decompressor.c:217 z_erofs_lz4_decompress_mem() warn: 'reason' isn't an ERR_PTR
fs/erofs/decompressor_lzma.c:193 z_erofs_lzma_decompress() warn: 'reason' is an error pointer or valid
fs/erofs/decompressor_zstd.c:180 z_erofs_zstd_decompress() warn: 'reason' is an error pointer or valid

fs/erofs/decompressor.c
    198 static int z_erofs_lz4_decompress_mem(struct z_erofs_decompress_req *rq, u8 *dst)
    199 {
    200         bool support_0padding = false, may_inplace = false;
    201         unsigned int inputmargin;
    202         u8 *out, *headpage, *src;
    203         const char *reason;
    204         int ret, maptype;
    205 
    206         DBG_BUGON(*rq->in == NULL);
    207         headpage = kmap_local_page(*rq->in);
    208 
    209         /* LZ4 decompression inplace is only safe if zero_padding is enabled */
    210         if (erofs_sb_has_zero_padding(EROFS_SB(rq->sb))) {
    211                 support_0padding = true;
    212                 reason = z_erofs_fixup_insize(rq, headpage + rq->pageofs_in,
    213                                 min_t(unsigned int, rq->inputsize,
    214                                       rq->sb->s_blocksize - rq->pageofs_in));
    215                 if (reason) {
    216                         kunmap_local(headpage);
--> 217                         return IS_ERR(reason) ? PTR_ERR(reason) : -EFSCORRUPTED;
    218                 }

The z_erofs_fixup_insize() function used to return error pointers, but
now it returns an error string or NULL.  So probably we could just
change this to:

		return -EFSCORRUPTED;

Are we planning to make it return error pointers again?

NULL means success in this case, right?  It's sort of weird how NULL means
success and a valid pointer means failure.

    219                 may_inplace = !((rq->pageofs_in + rq->inputsize) &
    220                                 (rq->sb->s_blocksize - 1));
    221         }

regards,
dan carpenter