library: Better PID file checks

library: Better PID file checks

This started off with fixing the compilier warning:

proc/readproc.c: In function ‘simple_nextpid’:
proc/readproc.c:1373:38: warning: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size 58 [-Wformat-truncation=]
 1373 |   snprintf(path, PROCPATHLEN, "/proc/%s", ent->d_name);
      |                                      ^~
proc/readproc.c:1373:3: note: ‘snprintf’ output between 7 and 262 bytes into a destination of size 64
 1373 |   snprintf(path, PROCPATHLEN, "/proc/%s", ent->d_name);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We know that ent->d_name will fit under 64 bytes because we check the
name starts with a digit. The first change was simple and changed the
printf to use tgid like the task function below it.

Is everything under /proc that starts with a digit a directory with a
PID only? Today, it is but there are no guarantees.

The entire function works ok if every non-pid directory doesn't start
with a number. We don't check for strtoul() having an issue nor
if the for loop just falls off the end. The moment the kernel guys
(or some module writer) think "/proc/12mykernelval" is a neat idea this
function is in trouble. We won't get buffer overflow as we are
using snprintf at least.

This change now:
 We check if strtoul() actually came across a number
 Process the pid directory as a conditional branch
 Treat falling off the for loop as a not-found

Signed-off-by: Craig Small <csmall@dropbear.xyz>