Re: [PATCH v5 3/3] mm: Dump the memory of slab object in kmem_dump_obj()

["Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>]

On 2023/8/3 18:34, Vlastimil Babka wrote:
> On 8/3/23 12:17, thunder.leizhen@huaweicloud.com wrote:
>> From: Zhen Lei <thunder.leizhen@huawei.com>
>>
>> The contents of the slab object may contain some magic words and other
>> useful information that may be helpful in locating problems such as
>> memory corruption and use-after-free.
>>
>> To avoid print flooding, dump up to "16 * sizeof(int) = 64" bytes
>> centered on argument 'ojbect'.
>>
>> For example:
>> slab kmalloc-64 start ffff4043802d8b40 pointer offset 24 size 64
>> [8b40]: 12345678 00000000 8092d000 ffff8000
>> [8b50]: 00101000 00000000 8199ee00 ffff4043
>> [8b60]: 00000000 00000000 00000000 00000100
>> [8b70]: 00000000 9abcdef0 a8744de4 ffffc7fe
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>> ---
>>  mm/slab_common.c | 30 +++++++++++++++++++++++++++---
>>  1 file changed, 27 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index ee6ed6dd7ba9fa5..0232de9a3b29cf5 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -553,7 +553,7 @@ static void kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *
>>  bool kmem_dump_obj(void *object)
>>  {
>>  	char *cp = IS_ENABLED(CONFIG_MMU) ? "" : "/vmalloc";
>> -	int i;
>> +	int i, object_size = 0;
>>  	struct slab *slab;
>>  	unsigned long ptroffset;
>>  	struct kmem_obj_info kp = { };
>> @@ -580,12 +580,36 @@ bool kmem_dump_obj(void *object)
>>  		ptroffset = ((char *)object - (char *)kp.kp_objp) - kp.kp_data_offset;
>>  		pr_cont(" pointer offset %lu", ptroffset);
>>  	}
>> -	if (kp.kp_slab_cache && kp.kp_slab_cache->object_size)
>> -		pr_cont(" size %u", kp.kp_slab_cache->object_size);
>> +	if (kp.kp_slab_cache && kp.kp_slab_cache->object_size) {
>> +		object_size = kp.kp_slab_cache->object_size;
>> +		pr_cont(" size %u", object_size);
>> +	}
>>  	if (kp.kp_ret)
>>  		pr_cont(" allocated at %pS\n", kp.kp_ret);
>>  	else
>>  		pr_cont("\n");
>> +
>> +	/* Dump a small piece of memory centered on 'object' */
>> +	if (kp.kp_objp && object_size) {
>> +		int *p = object, n = 16;
>> +
>> +		p += n / 2;
>> +		if ((void *)p > kp.kp_objp + object_size)
>> +			p = kp.kp_objp + object_size;
>> +
>> +		p -= n;
>> +		if ((void *)p < kp.kp_objp)
>> +			p = kp.kp_objp;
>> +
>> +		n = min_t(int, object_size / sizeof(int), n);
>> +		for (i = 0; i < n; i++, p++) {
>> +			if (i % 4 == 0)
>> +				pr_info("[%04lx]:", 0xffff & (unsigned long)p);
>> +			pr_cont(" %08x", *p);
>> +		}
>> +		pr_cont("\n");
> 
> There's a print_hex_dump() for this, see how it's used from e.g. __dump_page().

Thank you very much. The code has suddenly been a lot simpler.

However, print_hex_dump() can be further enhanced, I will add a patch, let's
discuss it together.

> 
> 
>> +	}
>> +
>>  	for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
>>  		if (!kp.kp_stack[i])
>>  			break;
> 
> .
> 

-- 
Regards,
  Zhen Lei