Re: [Crash-utility] [PATCH] Fix the "kmem -s" option for Linux 5.7 and later kernels

Hi, Kazu Thank you for the comment. On Mon, May 31, 2021 at 9:42 AM HAGIO KAZUHITO(萩尾　一仁) <k-hagio-ab(a)nec.com <mailto:k-hagio-ab@nec.com> > wrote: Hi Lianbo, > No, the patch fixes the above errors. Upstream kernel has changed the calculation formula in the > freelist_ptr() as below(marked it with "^^^^"), which added the "swab()" operation, crash also needs > to follow up this change, otherwise crash will get the error of "invalid freepointer". > Before: > crash> kmem -s > CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME > c0000000311a4200 152 0 0 0 64k fuse_request > c0000000311a8700 800 0 0 0 64k fuse_inode > c000000035df7600 528 0 0 0 64k xfs_dqtrx > c000000035df6600 496 0 0 0 64k xfs_dquot > kmem: xfs_buf: slab: c00c0000000b6500 invalid freepointer: 3808942d00004eb0 > kmem: xfs_buf: slab: c00c00000116ba00 invalid freepointer: b83fe85a040027b0 > ... Yes, you are right, and thanks for the info. I was confused because I could not reproduce the error message with a dozen vmcores. The above error messages are easily observed on ppc64le and s390 machines. With some debugging, I think maybe the message itself does not occur on x86_64 due to this issue, because get_freepointer() returns a value having 1 in bit 1 on x86_64 and breaks the loop below. @@ -19361,6 +19361,7 @@ get_freepointer(struct meminfo *si, void *object) return BADADDR; } + fprintf(fp, "DEBUG: vaddr: %lx ptr: %lx\n", vaddr, freelist_ptr(si,nextfree,vaddr)); return (freelist_ptr(si, nextfree, vaddr)); } crash> kmem -s CACHE OBJSIZE ALLOCATED TOTAL SLABS SSIZE NAME ... DEBUG: vaddr: ffff927b0501b5b8 ptr: b8b501057b92ec47 DEBUG: vaddr: ffff927b05221bb8 ptr: b81b22057b92f8c7 DEBUG: vaddr: ffff927b05218e38 ptr: 388e21057b92f647 DEBUG: vaddr: ffff927b052269b8 ptr: b86922057b92f647 ^bit1 = 1 ffff927b08452200 360 802 840 40 8k xfs_buf DEBUG: vaddr: ffff927b07991900 ptr: 1999077b92f1f7 DEBUG: vaddr: ffff927b017ac130 ptr: 30c17a017b92f017 DEBUG: vaddr: ffff927b0496d9c8 ptr: c8d996047b92fcc7 DEBUG: vaddr: ffff927b01bce5e0 ptr: e0e5bc017b92f37f ffff927b08452b00 200 50 480 24 4k xfs_bui_item for (q = freelist; q; q = get_freepointer(si, (void *)q)) { if (q & PAGE_MAPPING_ANON) break; <<-- break here However, the issue affects the calculation of allocated/free objects also on x86_64: -ffff927b08452200 360 802 840 40 8k xfs_buf +ffff927b08452200 360 777 840 40 8k xfs_buf ^^^ > > > Linux 5.7 and later kernels that contain kernel commit <1ad53d9fa3f6> > > > ("slub: improve bit diffusion for freelist ptr obfuscation") changed > > > the calculation formula in the freelist_ptr(), which added a swab() > > > call to mix bits a little more. When kernel is built with the > > > "CONFIG_SLAB_FREELIST_HARDENED=y",the "kmem -s" option fails with the > > > following errors, if there is no such patch. So, could I change the latter part of the commit message to the following? Yes, this also looks good.