util: do a better job of matching up pids with their binaries

This patch resolves: https://bugzilla.redhat.com/show_bug.cgi?id=871201

If libvirt is restarted after updating the dnsmasq or radvd packages,
a subsequent "virsh net-destroy" will fail to kill the dnsmasq/radvd
process.

The problem is that when libvirtd restarts, it re-reads the dnsmasq
and radvd pidfiles, then does a sanity check on each pid it finds,
including checking that the symbolic link in /proc/$pid/exe actually
points to the same file as the path used by libvirt to execute the
binary in the first place. If this fails, libvirt assumes that the
process is no longer alive.

But if the original binary has been replaced, the link in /proc is set
to "$binarypath (deleted)" (it literally has the string " (deleted)"
appended to the link text stored in the filesystem), so even if a new
binary exists in the same location, attempts to resolve the link will
fail.

In the end, not only is the old dnsmasq/radvd not terminated when the
network is stopped, but a new dnsmasq can't be started when the
network is later restarted (because the original process is still
listening on the ports that the new process wants).

The solution is, when the initial "use stat to check for identical
inodes" check for identity between /proc/$pid/exe and $binpath fails,
to check /proc/$pid/exe for a link ending with " (deleted)" and if so,
truncate that part of the link and compare what's left with the
original binarypath.

A twist to this problem is that on systems with "merged" /sbin and
/usr/sbin (i.e. /sbin is really just a symlink to /usr/sbin; Fedora
17+ is an example of this), libvirt may have started the process using
one path, but /proc/$pid/exe lists a different path (indeed, on F17
this is the case - libvirtd uses /sbin/dnsmasq, but /proc/$pid/exe
shows "/usr/sbin/dnsmasq"). The further bit of code to resolve this is
to call virFileResolveAllLinks() on both the original binarypath and
on the truncated link we read from /proc/$pid/exe, and compare the
results.

The resulting code still succeeds in all the same cases it did before,
but also succeeds if the binary was deleted or replaced after it was
started.
(cherry picked from commit 7bafe009)

cfg.mk

@@ -386,7 +386,7 @@ sc_prohibit_sprintf:
 	  $(_sc_search_regexp)
 
 sc_prohibit_readlink:
-	@prohibit='readlink *\('					\
+	@prohibit='\<readlink *\('					\
 	halt='use virFileResolveLink, not readlink'			\
 	  $(_sc_search_regexp)
 

src/util/virpidfile.c

@@ -35,6 +35,7 @@
 #include "logging.h"
 #include "virterror_internal.h"
 #include "c-ctype.h"
+#include "areadlink.h"
 
 #define VIR_FROM_THIS VIR_FROM_NONE
 
@@ -203,38 +204,91 @@ int virPidFileRead(const char *dir,
  */
 int virPidFileReadPathIfAlive(const char *path,
                               pid_t *pid,
-                              const char *binpath)
+                              const char *binPath)
 {
-    int rc;
-    char *procpath = NULL;
+    int ret, retPid;
+    bool isLink;
+    char *procPath = NULL;
+    char *procLink = NULL;
+    size_t procLinkLen;
+    char *resolvedBinPath = NULL;
+    char *resolvedProcLink = NULL;
+    const char deletedText[] = " (deleted)";
+    size_t deletedTextLen = strlen(deletedText);
+
+
+    /* only set this at the very end on success */
+    *pid = -1;
 
-    rc = virPidFileReadPath(path, pid);
-    if (rc < 0)
-        return rc;
+    if ((ret = virPidFileReadPath(path, &retPid)) < 0)
+        goto cleanup;
 
 #ifndef WIN32
     /* Check that it's still alive.  Safe to skip this sanity check on
      * mingw, which lacks kill().  */
-    if (kill(*pid, 0) < 0) {
-        *pid = -1;
-        return 0;
+    if (kill(retPid, 0) < 0) {
+        ret = 0;
+        retPid = -1;
+        goto cleanup;
     }
 #endif
 
-    if (binpath) {
-        if (virAsprintf(&procpath, "/proc/%lld/exe", (long long)*pid) < 0) {
-            *pid = -1;
-            return -1;
+    if (!binPath) {
+        /* we only knew the pid, and that pid is alive, so we can
+         * return it.
+         */
+        ret = 0;
+        goto cleanup;
     }
 
-        if (virFileIsLink(procpath) &&
-            virFileLinkPointsTo(procpath, binpath) == 0)
-            *pid = -1;
+    if (virAsprintf(&procPath, "/proc/%lld/exe", (long long)retPid) < 0) {
+        ret = -ENOMEM;
+        goto cleanup;
+    }
 
-        VIR_FREE(procpath);
+    if ((ret = virFileIsLink(procPath)) < 0)
+        goto cleanup;
+    isLink = ret;
+
+    if (isLink && virFileLinkPointsTo(procPath, binPath)) {
+        /* the link in /proc/$pid/exe is a symlink to a file
+         * that has the same inode as the file at binpath.
+         */
+        ret = 0;
+        goto cleanup;
     }
 
-    return 0;
+    /* Even if virFileLinkPointsTo returns a mismatch, it could be
+     * that the binary was deleted/replaced after it was executed. In
+     * that case the link in /proc/$pid/exe will contain
+     * "$procpath (deleted)".  Read that link, remove the " (deleted)"
+     * part, and see if it has the same canonicalized name as binpath.
+     */
+    if (!(procLink = areadlink(procPath))) {
+        ret = -errno;
+        goto cleanup;
+    }
+    procLinkLen = strlen(procLink);
+    if (procLinkLen > deletedTextLen)
+        procLink[procLinkLen - deletedTextLen] = 0;
+
+    if ((ret = virFileResolveAllLinks(binPath, &resolvedBinPath)) < 0)
+        goto cleanup;
+    if ((ret = virFileResolveAllLinks(procLink, &resolvedProcLink)) < 0)
+        goto cleanup;
+
+    ret = STREQ(resolvedBinPath, resolvedProcLink) ? 0 : -1;
+
+cleanup:
+    VIR_FREE(procPath);
+    VIR_FREE(procLink);
+    VIR_FREE(resolvedProcLink);
+    VIR_FREE(resolvedBinPath);
+
+    /* return the originally set pid of -1 unless we proclaim success */
+    if (ret == 0)
+        *pid = retPid;
+    return ret;
 }