Re: [PATCH] powerpc/kexec: Annotate umem_info members with __counted_by_ptr

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 03/10/26 14:23, Thorsten Blum wrote:
> On Fri, Sep 25, 2026 at 11:49:04AM +0530, Sourabh Jain wrote:
>> On 30/07/26 18:32, Thorsten Blum wrote:
>>> Add __counted_by_ptr() to umem_info::buf and umem_info::ranges to
>>> improve access bounds checking via CONFIG_UBSAN_BOUNDS and
>>> CONFIG_FORTIFY_SOURCE.
>>>
>>> Set the count fields before assigning the corresponding pointers, return
>>> early on krealloc() failure, and use sizeof(*buf) when deriving
>>> max_entries from the allocation size.
>>>
>>> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
>>> ---
>>>    arch/powerpc/kexec/file_load_64.c | 22 ++++++++++++----------
>>>    1 file changed, 12 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kexec/file_load_64.c b/arch/powerpc/kexec/file_load_64.c
>>> index 8c72e12ea44e..6424b668f0e9 100644
>>> --- a/arch/powerpc/kexec/file_load_64.c
>>> +++ b/arch/powerpc/kexec/file_load_64.c
>>> @@ -34,14 +34,15 @@
>>>    #include <asm/cputhreads.h>
>>>    struct umem_info {
>>> -	__be64 *buf;		/* data buffer for usable-memory property */
>>> +	/* data buffer for usable-memory property */
>>> +	__be64 *buf __counted_by_ptr(max_entries);
>>>    	u32 size;		/* size allocated for the data buffer */
>>>    	u32 max_entries;	/* maximum no. of entries */
>>>    	u32 idx;		/* index of current entry */
>>>    	/* usable memory ranges to look up */
>>>    	unsigned int nr_ranges;
>>> -	const struct range *ranges;
>>> +	const struct range *ranges __counted_by_ptr(nr_ranges);
>>>    };
>> [...]
>>>    const struct kexec_file_ops * const kexec_file_loaders[] = {
>>> @@ -83,11 +84,12 @@ static __be64 *check_realloc_usable_mem(struct umem_info *um_info, int cnt)
>>>    	new_size = um_info->size + MEM_RANGE_CHUNK_SZ;
>>>    	tbuf = krealloc(um_info->buf, new_size, GFP_KERNEL);
>>> -	if (tbuf) {
>>> -		um_info->buf = tbuf;
>>> -		um_info->size = new_size;
>>> -		um_info->max_entries = (um_info->size / sizeof(u64));
>>> -	}
>>> +	if (!tbuf)
>>> +		return NULL;
>>> +
>>> +	um_info->size = new_size;
>>> +	um_info->max_entries = um_info->size / sizeof(*um_info->buf);
>>> +	um_info->buf = tbuf;
>>
>> Could you please explain why size and max_entries are updated before the
>> buffer itself?
> __counted_by_ptr() requires the counter ->max_entries to be set before
> the ->buf pointer is assigned; otherwise you have an inconsistent state
> where the counter doesn't match the pointer.

But isn't updating the counter holding the buffer size before the actual 
buffer
can cause problems?

Can you share the document link of __counted_by_ptr() which says that 
size counter to be
updated before the buffer pointer.

Here is an example in fs/coredump.c file where size counter is updated 
after the buffer
with __counter_by_ptr():

https://github.com/torvalds/linux/blob/a90ee4305c4a5df72c11b31dacfdc76e00fcf78a/fs/coredump.c#L98
https://github.com/torvalds/linux/blob/a90ee4305c4a5df72c11b31dacfdc76e00fcf78a/fs/coredump.c#L115

- Sourabh Jain

> And ->size is moved up because ->max_entries depends on it.