On 2024/2/8 12:33, Eric Biggers wrote:
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.
Thank you for your commtents.
Although this behavior was not mentioned, it may indeed lead to
undefined performance, such as (we changed char [] to char *):
#include <sys/eventfd.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main()
{
//char str[32] = "hello world";
char *str = "hello world";
uint64_t value;
ssize_t size;
int fd;
fd = eventfd(0, 0);
size = write(fd, &str, strlen(str));
printf("eventfd: test writing a string:%s, size=%ld\n", str, size);
size = read(fd, &value, sizeof(value));
printf("eventfd: test reading as uint64, size=%ld, value=0x%lX\n",
size, value);
close(fd);
return 0;
}
$ ./a.out
eventfd: test writing a string:hello world, size=8
eventfd: test reading as uint64, size=8, value=0x560CC0134008
$ ./a.out
eventfd: test writing a string:hello world, size=8
eventfd: test reading as uint64, size=8, value=0x55A3CD373008
$ ./a.out
eventfd: test writing a string:hello world, size=8
eventfd: test reading as uint64, size=8, value=0x55B8D7B99008
--
Best wishes,
Wen
If the validation is indeed going to be made more strict, the manual page will
need to be fixed alongside it.
- Eric
