Re: [PATCH] eventfd: strictly check the count parameter of eventfd_write to avoid inputting illegal strings

[Eric Biggers]

On Wed, Feb 07, 2024 at 12:35:18AM +0800, wenyang.linux@xxxxxxxxxxx wrote:
> By checking whether count is equal to sizeof(ucnt), such errors
> could be detected. It also follows the requirements of the manual.

Does it?  This is what the eventfd manual page says:

     A write(2) fails with the error EINVAL if the size of the supplied buffer
     is less than 8 bytes, or if an attempt is made to write the value
     0xffffffffffffffff.

So, *technically* it doesn't mention the behavior if the size is greater than 8
bytes.  But one might assume that such writes are accepted, since otherwise it
would have been mentioned that they're rejected, just like writes < 8 bytes.

If the validation is indeed going to be made more strict, the manual page will
need to be fixed alongside it.

- Eric