gdb: revert std -> gdb unordered_map change in x86-nat.c

This particular change in commit 965deba1208 ("gdb: use
gdb::unordered_{set,map} at a few places") causes some regressions,
for instance when running gdb.base/catch-fork-kill.exp:

    (gdb) PASS: gdb.base/catch-fork-kill.exp: fork-kind=fork: exit-kind=exit: fork: continue to child fork
    continue^M
    Continuing.^M
    [Attaching after Thread 0x7ffff7e74740 (LWP 490949) fork to child process 490952]^M
    [New inferior 2 (process 490952)]^M
    [Thread debugging using libthread_db enabled]^M
    Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".^M
    =================================================================^M
    ^[[1m^[[31m==490919==ERROR: AddressSanitizer: heap-use-after-free on address 0x7c9bac051d88 at pc 0x55f46adc7583 bp 0x7ffc862a1e90 sp 0x7ffc862a1e80

Function x86_linux_nat_target::low_new_fork calls function
x86_debug_reg_state twice in a row.  The first one returns a reference
to an x86_debug_reg_state object in the map.  The second causes an
insertion to be made, moving the first object, rendering the first
reference invalid.

Revert the change and add a comment indicating that the location of
x86_debug_reg_state objects is important.

Change-Id: I85789d7657bec658a01908117efa25611e660878

diff --git a/gdb/x86-nat.c b/gdb/x86-nat.c
index 381364cdd70edeb1006aa4535e77a310f14b2909..43e7764f14271f242febc164590fcfdc0bb250ad 100644 (file)
--- a/gdb/x86-nat.c
+++ b/gdb/x86-nat.c
@@ -38,9 +38,13 @@ struct x86_dr_low_type x86_dr_low;
 /* Hash table storing per-process data.  We don't bind this to a
    per-inferior registry because of targets like x86 GNU/Linux that
    need to keep track of processes that aren't bound to any inferior
-   (e.g., fork children, checkpoints).  */
+   (e.g., fork children, checkpoints).
 
-static gdb::unordered_map<pid_t,
+   Use std::unordered_map rather than gdb::unordered_map, because the
+   location of entries must not change across two x86_debug_reg_state(ptid_t)
+   calls.  */
+
+static std::unordered_map<pid_t,
                          struct x86_debug_reg_state> x86_debug_process_state;
 
 /* See x86-nat.h.  */