(patching /usr/src/usr.bin/indent/indent.c on a HardenedBSD fork)

Hey, does this look robust to you?

static void bakcopy(void)
{
int n, bakchn, renamesuccess;
char buff[8 * 1024];
const char *p;

/* construct file name .Bfile */
for (p = in_name; *p; p++); /* skip to end of string */
while (p > in_name && *p != '/') /* find last '/' */
p--;
if (*p == '/')
p++;
sprintf(bakfile, "%s%s", p, simple_backup_suffix);

/* try first: move in_name to backup file; if that fails (and how?) then copy. */
if ((renamesuccess = rename(in_name, bakfile)) != 0) {
warn("could not make backup (%s -> %s) by moving; copying instead", in_name, bakfile);
/* copy in_name to backup file */
bakchn = creat(bakfile, 0600);
if (bakchn < 0)
err(1, "%s", bakfile);
while ((n = read(fileno(input), buff, sizeof(buff))) > 0)
if (write(bakchn, buff, n) != n)
err(1, "%s", bakfile);
if (n < 0)
err(1, "%s", in_name);
close(bakchn);
fclose(input);

/* re-open backup file as the input file */
input = fopen(bakfile, "r");
if (input == NULL)
err(1, "%s", bakfile);
}
/* now the original input file will be the output */
output = fopen(in_name, "w");
if (output == NULL) {
/* only if renamesuccess is -1 would we be able to unlink the backup without losing the file. */
if (renamesuccess == -1) {
unlink(bakfile);
err(1, "%s", in_name);
} else {
warn("fatal - unable to open %s", in_name);
if (rename(bakfile, in_name) == -1) err(1, "... and unable to rename backup file %s", bakfile);
exit(1); /* NOTREACHED unless rename succeeds */
}
}
}