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List-Unsubscribe: On Fri, Jul 25, 2025 at 06:10:51PM +0100, Matthew Wilcox wrote: > On Fri, Jul 25, 2025 at 06:47:01PM +0200, Vlastimil Babka wrote: > > On 7/25/25 08:49, Li Qiong wrote: > > > For debugging, object_err() prints free pointer of the object. > > > However, if check_valid_pointer() returns false for a object, > > > dereferncing `object + s->offset` can lead to a crash. Therefore, > > > print the object's address in such cases. > > I don't know where this patch came from (was it cc'd to linux-mm? i > don't see it) https://lore.kernel.org/all/20250725024854.1201926-1-liqiong@nfschina.com Looks like it's rejected by linux-mm for some reason.. > > > +/* > > > + * object - should be a valid object. > > > + * check_valid_pointer(s, slab, object) should be true. > > > + */ > > This comment is very confusing. It tries to ape kernel-doc style, > but if it were kernel-doc, the word before the hyphen should be the name > of the function, and it isn't. If we did use kernel-doc for this, we'd > use @object to denote that we're documenting the argument. Yes, the comment is indeed confusing and agree with your point. When I suggested it I expected adding something like: /* print_trailer() may deref invalid freepointer if object pointer is invalid */ WARN_ON_ONCE(!check_valid_pointer(s, slab, object)); to be added to object_err(). > But I don't see the need to pretend this is related to kernel-doc. This > would be better: > > /* > * 'object' must be a valid pointer into this slab. ie > * check_valid_pointer() would return true > */ > > I'm sure better wording for that is possible ... > > > > if (!check_valid_pointer(s, slab, object)) { > > > - object_err(s, slab, object, "Freelist Pointer check fails"); > > > + slab_err(s, slab, "Invalid object pointer 0x%p", object); > > > return 0; > > No, the error message is now wrong. It's not an object, it's the > freelist pointer. Because it's the object is about to be allocated, it will look like this: object pointer -> obj: [ garbage ][ freelist pointer ][ garbage ] SLUB uses check_valid_pointer() to check either 1) freelist pointer of an object is valid (e.g. in check_object()), or 2) an object pointer points to a valid address (e.g. in free_debug_processing()). In this case it's an object pointer, not a freelist pointer. Or am I misunderstanding something? > slab_err(s, slab, "Invalid freelist pointer %p", object); > > (the 0x%p is wrong because it will print 0x twice) "0x%p" is used all over the place in mm/slub.c. In the printk documentation [1]: > Plain Pointers > %p abcdef12 or 00000000abcdef12 0x%p should be 0xabcdef12 or 0x00000000abcdef12, no? [1] https://www.kernel.org/doc/html/next/core-api/printk-formats.html#printk-specifiers > But I think there are even more things wrong here. Like slab_err() is > not nerely as severe as slab_bug(), which is what used to be called. What do you mean by slab_err() is not as severe as slab_bug()? Both object_err() and slab_err() add a taint and trigger a WARNING. > And object_err() adds a taint, which this skips. adding a taint is done via slab_err()->__slab_err()->add_taint() -- Cheers, Harry / Hyeonggon