mdmfs: Fix many bugs in automatic md(4) creation.

[brooks]

Summary

This code allocated a correctly sized buffer, read past the end of the
source buffer, writing off the end of the target buffer, and then writing
a '\0' terminator past the end of the target buffer (in the wrong place).
It then leaked the buffer.

Switch to a statically sized buffer on the stack and update the source pointer and
length before use so the correct things are copied.

Fix a logic error in the checks that the format of the line is as
expected and move on out of an assert.

Remove an unneeded close(). fclose() closes the descriptor.

Found with: CheriABI
Obtained from: CheriBSD

[markj]

sbin/mdmfs/mdmfs.c

484 ↗

(On Diff #53694)

What if the line is just "md\n", i.e., there's no unit number? I think the second clause won't catch that case because linelen includes the newline character. That is, it looks like that should be linelen <= mdnamelen + 1.

[brooks]

• Make sure there's at least space for one digit of a unit number.

[jhb]

It's probably fine, but I wonder if it wouldn't be more readable to just use getline() instead. I think you'd end up with something like:

char *linebuf;
size_t linecap;
...
sfd = fdopen(...);
linebuf = NULL;
linecap = 0;
if (getline(&linebuf, &linecap, sfd) < mdnamelen ||
    strncmp(linebuf, mdname, mdnamelen) != 0)
        errx(1, "unexpected output from mdconfig (attach)");
ul = strtoul(linebuf + mdnamelen, &p, 10);
...
fclose(sfd);
free(linebuf);

sbin/mdmfs/mdmfs.c

490 ↗

(On Diff #53713)

This assertion is now pointless.

500 ↗

(On Diff #53713)

This close() is wrong, but harmless. fdopen() "takes" the fd and so fclose() already closes it.

[kib]

The buffer is not statically allocated, it is statically sized (in the revision summary).

[brooks]

• Remove obsolete and always true assert.
• Remove pointless close(). fclose() closes the descriptor.