linux: switch to target process UID/GID

Ensure access to /tmp when overlayfs involved.

Author: mdavidsaver

ci_core_dumper/linux.py

@@ -294,6 +294,22 @@ def dump(outdir, gdb, extra_cmds):
 
 def dump2(pid, gdb, extra_cmds):
     # running as root, fully in the target/container namespaces
+    print('Initial root is uid %d, gid %d'%(os.getuid(), os.getgid()))
+
+    uid = gid = None
+    with open('/proc/{}/status'.format(pid)) as F:
+        for line in F:
+            if line.startswith('Uid:'): # Uid:    1000    1000    1000    1000
+                uid = int(line.split()[1])
+
+            elif line.startswith('Gid:'): # Gid:    1000    1000    1000    1000
+                uid = int(line.split()[1])
+
+    if gid:
+        os.setegid(gid)
+    if uid:
+        os.seteuid(uid)
+    print('Target UID %d/%d'%(uid, gid))
 
     # os.environ is still from the init namespaces.
     # copy the target process environment