Messages in this thread | | | Date | Thu, 8 Feb 2024 13:34:45 +0800 | Subject | Re: [PATCH] eventfd: strictly check the count parameter of eventfd_write to avoid inputting illegal strings | From | Wen Yang <> |
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On 2024/2/8 12:33, Eric Biggers wrote: > On Wed, Feb 07, 2024 at 12:35:18AM +0800, wenyang.linux@foxmail.com 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
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