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

[Wen Yang]

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