dwarf_loader: Use libdw__lock for dwarf_getlocation(s)

Eduard noticed [1] intermittent segmentation faults triggered by caching
done internally in dwarf_getlocation(s).

A binary tree of location information is cached in the CU, and if
multiple threads access it concurrently we can get segmentation faults.

It is possible to compile elfutils with experimental thread-safe
support, but safer for now to add locking to pahole.

No additional overhead in pahole encoding was observed
as a result of these changes.

[1] https://lore.kernel.org/dwarves/[email protected]/

Reported-by: Eduard Zingerman <[email protected]>
Suggested-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Alan Maguire <[email protected]>
Tested-by: Jiri Olsa <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

dwarf_loader.c

@@ -450,7 +450,14 @@ static bool attr_type(Dwarf_Die *die, uint32_t attr_name, Dwarf_Off *offset)
 static int attr_location(Dwarf_Die *die, Dwarf_Op **expr, size_t *exprlen)
 {
 	Dwarf_Attribute attr;
+	int ret = 1;
+
 	if (dwarf_attr(die, DW_AT_location, &attr) != NULL) {
+		/* use libdw__lock as dwarf_getlocation(s) has concurrency
+		 * issues when libdw is not compiled with experimental
+		 * --enable-thread-safety
+		 */
+		pthread_mutex_lock(&libdw__lock);
 		if (dwarf_getlocation(&attr, expr, exprlen) == 0) {
 			/* DW_OP_addrx needs additional lookup for real addr. */
 			if (*exprlen != 0 && expr[0]->atom == DW_OP_addrx) {
@@ -462,11 +469,12 @@ static int attr_location(Dwarf_Die *die, Dwarf_Op **expr, size_t *exprlen)
 
 				expr[0]->number = address;
 			}
-			return 0;
+			ret = 0;
 		}
+		pthread_mutex_unlock(&libdw__lock);
 	}
 
-	return 1;
+	return ret;
 }
 
 /* The struct dwarf_tag has a fixed size while the 'struct tag' is just the base
@@ -1194,6 +1202,10 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 	int loc_num = -1;
 	int ret = -1;
 
+	/* use libdw__lock as dwarf_getlocation(s) has concurrency issues
+	 * when libdw is not compiled with experimental --enable-thread-safety
+	 */
+	pthread_mutex_lock(&libdw__lock);
 	while ((offset = __dwarf_getlocations(attr, offset, &base, &start, &end, &expr, &exprlen)) > 0) {
 		loc_num++;
 
@@ -1230,6 +1242,7 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 		}
 	}
 out:
+	pthread_mutex_unlock(&libdw__lock);
 	return ret;
 }