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Message-ID: <tencent_1D3AC867D2233D8E19C8CFF3B9A8AA893A05@qq.com>
Date: Thu, 8 Feb 2024 13:34:45 +0800
From: Wen Yang <wenyang.linux@...mail.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Christian Brauner <brauner@...nel.org>, Jens Axboe <axboe@...nel.dk>,
Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
David Woodhouse <dwmw@...zon.co.uk>, Matthew Wilcox <willy@...radead.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: strictly check the count parameter of
eventfd_write to avoid inputting illegal strings
On 2024/2/8 12:33, Eric Biggers wrote:
> On Wed, Feb 07, 2024 at 12:35:18AM +0800, wenyang.linux@...mail.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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