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SubjectRe: [PATCH] fs: Fix truncate never updates m/ctime
Date
> On Mon, Nov 15, 2021 at 07:00:18PM +0800, Jubin Zhong wrote:
>> From: zhongjubin <zhongjubin@huawei.com>
>>
>> Syscall truncate() never updates m/ctime even if the file size is
>> changed. However, this is incorrect according to man file:
>>
>> truncate (2):
>> If the size changed, then the st_ctime and st_mtime fields
>> (respectively, time of last status change and time of last modification;
>> see stat(2)) for the file are updated, and the set-user-ID and
>> set-group-ID mode bits may be cleared.
>>
>> Check file size before do_truncate() to fix this.
>
> Please try to actually reproduce your alleged "bug". And maybe also
> look at the actual setattr implementations. Hint: The XFS one even
> has extensive comments.

Thanks for your advice. I found this problem on yaffs2 in the beginning,
ftruncate() always works fine but truncate() does not. Now I have done
a few more tests and the following are the results:

Test Environmont:
kernel: Linux Kernel v5.16
hardware: QEMU emulator version 3.1.0
arch: vexpress-v2p-ca9

Teset Results:
filesystems m/ctime updated by truncate?
jffs2 fail
yaffs2 fail
ubifs success
ext2 success
ext4 success
tmpfs success
xfs success

Test Steps:
1. cd /path/to/mnt/point
2. dd if=/dev/zero of=test bs=1M count=1
3. stat test
4. /bin/my_truncate -s 1024 test
5. stat test
6. compare m/ctime of step 5 with step 3

Program source:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
int main(int argc, char **argv)
{
int ret;
char file_name[128] = {0};

if (argc < 4 || argv == NULL || argv[1] == NULL || argv[2] == NULL || argv[3] == NULL) {
return -1;
}

if (strcmp(argv[1], "-s")) {
return -1;
}

if (realpath(argv[3], file_name) == NULL) {
printf("truncate: input file name %s err.\n", argv[3]);
return -1;
}

off_t size = (off_t)strtol(argv[2], 0, 0);
ret = truncate(file_name, size);
if (ret) {
printf("truncate return err %d\n", ret);
}
return ret;
}

I work on embedded devices so concern about jffs2/yaffs2/ubifs the most.
If there are any errors in my test program please let me know.

Thanks.

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