Re: Subject: [PATCH v2] slab: kmalloc_size_roundup() must not return 0 for non-zero size

[Vlastimil Babka <vbabka@suse.cz>]

On 9/20/23 12:06, David Laight wrote:
> From: Vlastimil Babka
>> Sent: 20 September 2023 10:58
>> 
>> On 9/11/23 18:38, David Laight wrote:
>> >> >> So perhaps the best would be to return size for c == NULL, but also do a
>> >> >> WARN_ONCE?
>> >> >
>> >> > That would add a real function call to an otherwise leaf function
>> >> > and almost certainly require the compiler create a stack frame.
>> >>
>> >> Hm I thought WARN is done by tripping on undefined instruction like BUG
>> >> these days. Also any code that accepts the call to kmalloc_size_roundup
>> >> probably could accept that too.
>> >
>> > It's probably just worth removing the c == NULL check and
>> > assuming there won't be any fallout.
>> > The NULL pointer deref is an easy to debug as anything else.
>> >
>> > If it gets called in any early init code it'll soon show up.
>> 
>> Good point, early crash should be ok.
>> So how about this with my tweaks, looks ok?
> 
> Is that just/mainly the change to assume that kmalloc_slab() doesn't fail?

Yes.

> You can also remove 'c'.
> 	return kmalloc_slab(size, GFP_KERNEL, 0)->object_size;
> isn't too long.

Right, did that and pushed to -next. Thanks!

> I also did a grep for callers.
> Nothing in early code, IIRC mainly 'net'.
> 
> 	David
> 
>> I'll put it in -next and
>> send with another hotfix to 6.6 next week.
>> ----8<----
>> From f5de1ee7b35d7ab35c21c79dd13cea49fbe239b7 Mon Sep 17 00:00:00 2001
>> From: David Laight <david.laight@aculab.com>
>> Date: Thu, 7 Sep 2023 12:42:20 +0000
>> Subject: [PATCH] Subject: [PATCH v2] slab: kmalloc_size_roundup() must not
>>  return 0 for non-zero size
>> 
>> The typical use of kmalloc_size_roundup() is:
>> 
>> 	ptr = kmalloc(sz = kmalloc_size_roundup(size), ...);
>> 	if (!ptr) return -ENOMEM.
>> 
>> This means it is vitally important that the returned value isn't less
>> than the argument even if the argument is insane.
>> In particular if kmalloc_slab() fails or the value is above
>> (MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return
>> its single zero-length buffer ZERO_SIZE_PTR.
>> 
>> Fix this by returning the input size if the size exceeds
>> KMALLOC_MAX_SIZE. kmalloc() will then return NULL as the size really is
>> too big.
>> 
>> kmalloc_slab() should not normally return NULL, unless called too early.
>> Again, returning zero is not the correct action as it can be in some
>> usage scenarios stored to a variable and only later cause kmalloc()
>> return ZERO_SIZE_PTR and subsequent crashes on access. Instead we can
>> simply stop checking the kmalloc_slab() result completely, as calling
>> kmalloc_size_roundup() too early would then result in an immediate crash
>> during boot and the developer noticing an issue in their code.
>> 
>> [vbabka@suse.cz: remove kmalloc_slab() result check, tweak comments and
>>  commit log]
>> Signed-off-by: David Laight <david.laight@aculab.com>
>> Fixes: 05a940656e1e ("slab: Introduce kmalloc_size_roundup()")
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/slab_common.c | 25 ++++++++++++++-----------
>>  1 file changed, 14 insertions(+), 11 deletions(-)
>> 
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index e99e821065c3..1dc108224bd1 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -747,22 +747,25 @@ size_t kmalloc_size_roundup(size_t size)
>>  {
>>  	struct kmem_cache *c;
>> 
>> -	/* Short-circuit the 0 size case. */
>> -	if (unlikely(size == 0))
>> -		return 0;
>> -	/* Short-circuit saturated "too-large" case. */
>> -	if (unlikely(size == SIZE_MAX))
>> -		return SIZE_MAX;
>> +	if (size && size <= KMALLOC_MAX_CACHE_SIZE) {
>> +		/*
>> +		 * The flags don't matter since size_index is common to all.
>> +		 * Neither does the caller for just getting ->object_size.
>> +		 */
>> +		c = kmalloc_slab(size, GFP_KERNEL, 0);
>> +		return c->object_size;
>> +	}
>> +
>>  	/* Above the smaller buckets, size is a multiple of page size. */
>> -	if (size > KMALLOC_MAX_CACHE_SIZE)
>> +	if (size && size <= KMALLOC_MAX_SIZE)
>>  		return PAGE_SIZE << get_order(size);
>> 
>>  	/*
>> -	 * The flags don't matter since size_index is common to all.
>> -	 * Neither does the caller for just getting ->object_size.
>> +	 * Return 'size' for 0 - kmalloc() returns ZERO_SIZE_PTR
>> +	 * and very large size - kmalloc() may fail.
>>  	 */
>> -	c = kmalloc_slab(size, GFP_KERNEL, 0);
>> -	return c ? c->object_size : 0;
>> +	return size;
>> +
>>  }
>>  EXPORT_SYMBOL(kmalloc_size_roundup);
>> 
>> --
>> 2.42.0
>> 
>> 
> 
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