sysctl_writes_strict documentation + an oddity?

From: Michael Kerrisk (man-pages)
Date: Sat May 09 2015 - 04:54:34 EST

Hi Kees,

I discovered that you added /proc/sys/kernel/sysctl_writes_strict in
Linux 3.16. In passing, I'll just mention that was an API change that
should have been CCed to linux-api@xxxxxxxxxxxxxxxx

Anyway, I've tried to write this file up for the proc(5) man page,
and I have two requests:

1) Could you review this text?
2) I've found some behavior that surprised me, and I am wondering if it
is intended. Could you let me know your thoughts?

===== 1) man-page text =====

The man-page text, heavily based on your text in
Documentation/sysctl/kernel.txt, is as follows:

/proc/sys/kernel/sysctl_writes_strict (since Linux 3.16)
The value in this file determines how the file offset
affects the behavior of updating entries in files under
/proc/sys. The file has three possible values:

-1 This provides legacy handling, with no printk warnâ
ings. Each write(2) must fully contain the value to
be written, and multiple writes on the same file
descriptor will overwrite the entire value, regardless
of the file position.

0 (default) This provides the same behavior as for -1,
but printk warnings are written for processes that
perform writes when the file offset is not 0.

1 Respect the file offset when writing strings into
/proc/sys files. Multiple writes will append to the
value buffer. Anything written beyond the maximum
length of the value buffer will be ignored. Writes to
numeric /proc/sys entries must always be at file offâ
set 0 and the value must be fully contained in the
buffer provided to write(2).

===== 2) Behavior puzzle (a) =====

The last sentence quoted from the man page was based on your sentence

Writes to numeric sysctl entries must always be at file position 0
and the value must be fully contained in the buffer sent in the write

So, I had interpreted /proc/sys/kernel/sysctl_writes_strict==1 to
mean that if one writes into a numeric /proc/sys file at an offset
other than zero, the write() will fail with some kind of error.
But this seems not to be the case. Instead, the write() succeeds,
but the file is left unmodified. That's surprising, I find. So, I'm
wondering whether the implementation deviates from your intention.

There's a test program below, which takes arguments as follows

./a.out pathname offset string

And here's a test run that demonstrates the behavior:

$ sudo sh -c "echo 1 > /proc/sys/kernel/sysctl_writes_strict"
$ cat /proc/sys/kernel/pid_max
$ sudo dmesg --clear
$ sudo ./a.out /proc/sys/kernel/pid_max 1 3000
write() succeeded (return value 4)
$ cat /proc/sys/kernel/pid_max
$ dmesg

As you can see above, an attempt was made to write into the
/proc/sys/kernel/pid_max file at offset 1.
The write() returned successfully (reporting 4 bytes written)
but the file contents were unchanged, and no printk() warning
was issued. Is this intended behavior?

===== 2) Behavior puzzle (b) =====

In commit f88083005ab319abba5d0b2e4e997558245493c8, there is this note:

This adds the sysctl kernel.sysctl_writes_strict to control the write
behavior. The default (0) reports when VFS position is non-0 on a
write, but retains legacy behavior, -1 disables the warning, and 1
enables the position-respecting behavior.

The long-term plan here is to wait for userspace to be fixed in response
to the new warning and to then switch the default kernel behavior to the
new position-respecting behavior.

(That last para was added to the commit message by AKPM, I see.)

But, I wonder here whether /proc/sys/kernel/sysctl_writes_strict==0
is going to help with the long-term plan. The problem is that in
warn_sysctl_write(), pr_warn_once() is used. This means that only
the first offending user-space application that writes to *any*
/proc/sys file will generate the printk warning. If that application
isn't fixed, then none of the other "broken" applications will be
discovered. It therefore seems possible that it could be a very long
time before we could "switch the default kernel behavior to the
new position-respecting behavior".

Looking over old mails
I see that you're aware of the problem, but it seems to me that
the switch to pr_warn_once() (for fear of spamming the log) likely
dooms the long-term plan to failure. Your thoughts?




#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

main(int argc, char *argv[])
char *pathname;
off_t offset;
char *string;
int fd;
ssize_t numWritten;

if (argc != 4) {
fprintf(stderr, "Usage: %s pathname offset string\n", argv[0]);

pathname = argv[1];
offset = strtoll(argv[2], NULL, 0);
string = argv[3];

fd = open(pathname, O_RDWR);
if (fd == -1)

if (lseek(fd, offset, SEEK_SET) == -1)

numWritten = write(fd, string, strlen(string));
if (numWritten == -1)

printf("write() succeeded (return value %zd)\n", numWritten);


Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
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