Messages in this thread | | | Date | Wed, 10 Jan 2024 23:06:22 +0100 | Subject | Re: [PATCH v2] workqueue.c: Increase workqueue name length | From | Rasmus Villemoes <> |
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On 10/01/2024 22.52, Rafael Aquini wrote: > On Wed, Jan 10, 2024 at 09:47:56PM +0100, Rasmus Villemoes wrote: >> On 10/01/2024 21.29, Audra Mitchell wrote: >> >>> @@ -4663,9 +4663,10 @@ struct workqueue_struct *alloc_workqueue(const char *fmt, >>> unsigned int flags, >>> int max_active, ...) >>> { >>> - va_list args; >>> + va_list args, args_copy; >>> struct workqueue_struct *wq; >>> struct pool_workqueue *pwq; >>> + int len; >>> >>> /* >>> * Unbound && max_active == 1 used to imply ordered, which is no longer >>> @@ -4692,6 +4693,13 @@ struct workqueue_struct *alloc_workqueue(const char *fmt, >>> } >>> >>> va_start(args, max_active); >>> + va_copy(args_copy, args); >>> + len = vsnprintf(NULL, 0, fmt, args_copy); >>> + WARN(len > WQ_NAME_LEN, >>> + "workqueue: wq->name too long (%d). Truncated to WQ_NAME_LEN (%d)\n", >>> + len, WQ_NAME_LEN); >>> + >>> + va_end(args_copy); >>> vsnprintf(wq->name, sizeof(wq->name), fmt, args); >> >> Eh, why not just _not_ throw away the return value from the existing >> vsnprintf() and do "len >= sizeof(wq->name)" to know if truncation >> happened? There's really no need need to do vsnprintf() twice. (And yes, >> you want >=, not >). >> > > The extra vsnprintf call is required because the return of the existing > vsnprintf() is going to be already capped by sizeof(wq->name).
No, it is not. vsnprintf() returns the length of the would-be-created string if the buffer was big enough. That is independent of whether one does a dummy NULL,0 call or just calls it with a real, but possibly too small, buffer.
This is true for userspace (as required by posix) as well as the kernel implementation of vsnprintf(). What makes you think otherwise?
The kernel _also_ happens to have a non-standardized function called vscnprintf (note the c) which returns the possibly-truncated result. But that's irrelevant here.
>> Oh, and definitely not WARN, pr_warn() or pr_warn_once() please. >> > > Then you lose the ability to figure out what was trying to create the > wq with the inflated name. Also, the _once variants don't seem to do > good here, because alloc_workqueue() can be called by different > drivers.
I assume that whatever creates the wq will do so on every boot, and the name is most likely some fixed thing. So you're essentially setting up some configurations to do a WARN on every single boot, not to mention that for some machines that implies a panic... It really is not something that warrants a WARN.
As for figuring out what caused that too-long name, well, I'd hope that the 31 meaningful bytes that did get produced would provide a sufficiently good hint.
Rasmus
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