linux-kernel - Re: [PATCH v3 2/3] treewide: Switch memcpy() users of 'task->comm' to a more safer implementation

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Message-ID: <4af48ad5-1aa7-46d0-bfca-7779294e355c@igalia.com>
Date: Thu, 8 May 2025 13:47:39 +0530
From: Bhupesh Sharma <bhsharma@...lia.com>
To: Petr Mladek <pmladek@...e.com>, Bhupesh <bhupesh@...lia.com>
Cc: akpm@...ux-foundation.org, kernel-dev@...lia.com,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
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Subject: Re: [PATCH v3 2/3] treewide: Switch memcpy() users of 'task->comm' to
 a more safer implementation

Hi Petr,

On 5/7/25 5:59 PM, Petr Mladek wrote:
> On Wed 2025-05-07 16:34:43, Bhupesh wrote:
>> As Linus mentioned in [1], currently we have several memcpy() use-cases
>> which use 'current->comm' to copy the task name over to local copies.
>> For an example:
>>
>>   ...
>>   char comm[TASK_COMM_LEN];
>>   memcpy(comm, current->comm, TASK_COMM_LEN);
>>   ...
>>
>> These should be modified so that we can later implement approaches
>> to handle the task->comm's 16-byte length limitation (TASK_COMM_LEN)
>> is a more modular way (follow-up patches do the same):
>>
>>   ...
>>   char comm[TASK_COMM_LEN];
>>   memcpy(comm, current->comm, TASK_COMM_LEN);
>>   comm[TASK_COMM_LEN - 1] = 0;
>>   ...
>>
>> The relevant 'memcpy()' users were identified using the following search
>> pattern:
>>   $ git grep 'memcpy.*->comm\>'
>>
>> [1]. https://lore.kernel.org/all/CAHk-=wjAmmHUg6vho1KjzQi2=psR30+CogFd4aXrThr2gsiS4g@mail.gmail.com/
>>
>> --- a/include/linux/coredump.h
>> +++ b/include/linux/coredump.h
>> @@ -53,7 +53,8 @@ extern void do_coredump(const kernel_siginfo_t *siginfo);
>>   	do {	\
>>   		char comm[TASK_COMM_LEN];	\
>>   		/* This will always be NUL terminated. */ \
>> -		memcpy(comm, current->comm, sizeof(comm)); \
>> +		memcpy(comm, current->comm, TASK_COMM_LEN); \
>> +		comm[TASK_COMM_LEN] = '\0'; \
> I would expect that we replace this with a helper function/macro
> which would do the right thing.
>
> Why is get_task_comm() not used here, please?
>
>>   		printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n",	\
> Also the name seems to be used for printing a debug information.
> I would expect that we could use the bigger buffer here and print
> the "full" name. Is this planed, please?
>
>>   			task_tgid_vnr(current), (int)strlen(comm), comm, ##__VA_ARGS__);	\
>>   	} while (0)	\
>> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
>> index bd0ea07338eb..94a941ac2034 100644
>> --- a/include/trace/events/block.h
>> +++ b/include/trace/events/block.h
>> @@ -214,6 +214,7 @@ DECLARE_EVENT_CLASS(block_rq,
>>   		blk_fill_rwbs(__entry->rwbs, rq->cmd_flags);
>>   		__get_str(cmd)[0] = '\0';
>>   		memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
>> +		__entry->comm[TASK_COMM_LEN - 1] = '\0';
> Same for all other callers.
>
> That said, I am not sure if the larger buffer is save in all situations.
>
>>   	),
>

Thanks for the review, I agree on using the helper / wrapper function to 
replace this open-coded memcpy + set last entry as '\0'.

However I see that Steven has already shared a RFC approach (see [1]), 
to use __string() instead of fixed lengths for 'task->comm' for tracing 
events.
I plan to  rebase my v4 on top of his RFC, which might mean that this 
patch would no longer be needed in the v4.

[1]. 
https://lore.kernel.org/linux-trace-kernel/20250507133458.51bafd95@gandalf.local.home/

Regards,
Bhupesh