Introduce kmem_scnprintf()

`snprintf()` is meant to protect against buffer overflows, but operating
on the buffer using its return value, possibly by calling it again, can
cause a buffer overflow, because it will return how many characters it
would have written if it had enough space even when it did not. In a
number of places, we repeatedly call snprintf() by successively
incrementing a buffer offset and decrementing a buffer length, by its
return value. This is a potentially unsafe usage of `snprintf()`
whenever the buffer length is reached. CodeQL complained about this.

To fix this, we introduce `kmem_scnprintf()`, which will return 0 when
the buffer is zero or the number of written characters, minus 1 to
exclude the NULL character, when the buffer was too small. In all other
cases, it behaves like snprintf(). The name is inspired by the Linux and
XNU kernels' `scnprintf()`. The implementation was written before I
thought to look at `scnprintf()` and had a good name for it, but it
turned out to have identical semantics to the Linux kernel version.
That lead to the name, `kmem_scnprintf()`.

CodeQL only catches this issue in loops, so repeated use of snprintf()
outside of a loop was not caught. As a result, a thorough audit of the
codebase was done to examine all instances of `snprintf()` usage for
potential problems and a few were caught. Fixes for them are included in
this patch.

Unfortunately, ZED is one of the places where `snprintf()` is
potentially used incorrectly. Since using `kmem_scnprintf()` in it would
require changing how it is linked, we modify its usage to make it safe,
no matter what buffer length is used. In addition, there was a bug in
the use of the return value where the NULL format character was not
being written by pwrite(). That has been fixed.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Richard Yao <[email protected]>
Closes #14098

1 files changed, 5 insertions, 1 deletions

diff --git a/cmd/zed/zed_event.c b/cmd/zed/zed_event.c
index 079c6a043..ebd6851a2 100644
--- a/cmd/zed/zed_event.c
+++ b/cmd/zed/zed_event.c
@@ -139,8 +139,12 @@ _bump_event_queue_length(void)
 	if (qlen == orig_qlen)
 		goto done;
 	wr = snprintf(qlen_buf, sizeof (qlen_buf), "%ld", qlen);
+	if (wr >= sizeof (qlen_buf)) {
+		wr = sizeof (qlen_buf) - 1;
+		zed_log_msg(LOG_WARNING, "Truncation in %s()", __func__);
+	}
 
-	if (pwrite(zzlm, qlen_buf, wr, 0) < 0)
+	if (pwrite(zzlm, qlen_buf, wr + 1, 0) < 0)
 		goto done;
 
 	zed_log_msg(LOG_WARNING, "Bumping queue length to %ld", qlen);