[PATCH v2] dtprobed: use /proc/$pid/map_files, not the filename of the mapping

[Nick Alcock <nick.alcock@oracle.com>]

Instead of using prf->prf_mapname (which resolves to the mapped file's
target), use Pmap_mapfile_name() to get the actual
/proc/$pid/map_files/* path.  These magic symlinks can be opened even
when their corresponding files are deleted or in an inaccessible
filesystem namespace, ensuring we can read the mapping contents
reliably.  DTrace already does this to read mappings during USDT probe
lookup.

Fixes issues with probes in paths like /home when dtprobed
is sandboxed by systemd.

Tested on both systemd and non-systemd (non-jailed) systems, with USDT
programs running out of /tmp, /usr/local and /home.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
---
 dtprobed/dtprobed.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dtprobed/dtprobed.c b/dtprobed/dtprobed.c
index a808586559d96..9a6928055cd13 100644
--- a/dtprobed/dtprobed.c
+++ b/dtprobed/dtprobed.c
@@ -487,7 +487,7 @@ handle_usdt_notes(pid_t pid, uintptr_t addr)
 		fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapping (process dead?)\n",
 			 pid);
 		goto out;
-	} else if ((fn = prf->prf_mapname) == NULL) {
+	} else if ((fn = Pmap_mapfile_name(P, mapp)) == NULL) {
 		fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapname (process dead?)\n",
 			 pid);
 		goto out;

base-commit: 6e94c7d0a253806f85c39ff5f4e32a800d4cb6b6
-- 
2.52.0.285.g44a719c714.dirty