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 <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14098

include/sys/spa.h

@@ -600,7 +600,7 @@ typedef struct blkptr {
 
 /*
  * This macro allows code sharing between zfs, libzpool, and mdb.
- * 'func' is either snprintf() or mdb_snprintf().
+ * 'func' is either kmem_scnprintf() or mdb_snprintf().
  * 'ws' (whitespace) can be ' ' for single-line format, '\n' for multi-line.
  */
 

include/sys/zfs_context.h

@@ -695,6 +695,9 @@ extern char *kmem_asprintf(const char *fmt, ...);
 #define	kmem_strfree(str) kmem_free((str), strlen(str) + 1)
 #define	kmem_strdup(s)  strdup(s)
 
+extern int kmem_scnprintf(char *restrict str, size_t size,
+    const char *restrict fmt, ...);
+
 /*
  * Hostname information
  */