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SubjectRe: [PATCH v2] printk: Do not lose last line in kmsg buffer dump
On Fri, Jul 12, 2019 at 10:09:04AM +0200, Petr Mladek wrote:
> The patch looks like a hack using a hole that the next cycle
> does not longer check the number of really stored characters.
>
> What would happen when msg_print_text() starts adding
> the trailing '\0' as suggested by
> https://lkml.kernel.org/r/20190710121049.rwhk7fknfzn3cfkz@pathway.suse.cz

I did have a look at that possibility, but I didn't see how that could
work without potentially affecting userspace users of the syslog ABI.
AFAICS the suggested change in msg_print_text() can be done in one of
three ways:

(1) msg_print_text() adds the '\0' and includes this length both when
it estimates the size (NULL buffer) and when it actually prints:

If we do this:
- kmsg_dump_get_line_nolock() would have to subtract 1 from the len
since its callers expected that len is always smaller than the
size of the buffer.
- The buffers given to use via the syslog interface will now include
a '\0', potentially affecting userspace applications which use
this ABI.

(2) msg_print_text() adds the '\0', and includes this in the length
only when estimating the size, and not when it actually prints.

If we do this:
- SYSLOG_ACTION_SIZE_UNREAD tries uses the size estimate to give
userspace a count of how many characters are present in the
buffer, and now this count will start differing from the actual
count that can be read, potentially affecting userspace
applications.

(3) msg_print_text() adds the '\0', and does not include this length
in the result at all.

If we do this:
- The original kmsg dump issue is not solved, since the last line
is still lost.

> BTW: What is the motivation for this fix? Is a bug report
> or just some research of possible buffer overflows?

The fix is not attempting to fix a buffer overflow, theoretical or
otherwise.

It's a fix for a bug in functionality which has been observed on our
systems: We use pstore to save the kernel log when the kernel crashes,
and sometimes the log in the pstore misses the last line, and since the
last line usual says why we're panicing so it's rather important not to
miss.

> The commit message pretends that the problem is bigger than
> it really is. It is about one byte and not one line.

I'm not quite sure I follow. The current code does fail to include the
*entire* last line.

The memcpy on line #1294 is never executed for the last line because we
stop the loop because of the check on line #1289:

1270 static size_t msg_print_text(const struct printk_log *msg, bool syslog, char *buf, size_t size)
1271 {
1272 const char *text = log_text(msg);
1273 size_t text_size = msg->text_len;
1274 size_t len = 0;
1275
1276 do {
1277 const char *next = memchr(text, '\n', text_size);
1278 size_t text_len;
1279
1280 if (next) {
1281 text_len = next - text;
1282 next++;
1283 text_size -= next - text;
1284 } else {
1285 text_len = text_size;
1286 }
1287
1288 if (buf) {
1289 if (print_prefix(msg, syslog, NULL) +
1290 text_len + 1 > size - len)
1291 break;
1292
1293 len += print_prefix(msg, syslog, buf + len);
1294 memcpy(buf + len, text, text_len);
1295 len += text_len;
1296 buf[len++] = '\n';
1297 } else {
1298 /* SYSLOG_ACTION_* buffer size only calculation */
1299 len += print_prefix(msg, syslog, NULL);
1300 len += text_len;
1301 len++;
1302 }

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