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authorMarkus Metzger <markus.t.metzger@intel.com>2016-10-14 09:08:01 +0200
committerMarkus Metzger <markus.t.metzger@intel.com>2016-11-14 10:15:42 +0100
commitdb58b3735f45345c06cb9a14d0f83f5b26c1ebf3 (patch)
treecbcf6405feb64c5e7ba7f141d739b7f1036a9e8d /gdb
parent80bb3407f6891aff5dbed5b2fd5c3dae8f227319 (diff)
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btrace: read entire aux buffer
The data_head of a perf event data buffer grows indefinitely. Users are expected to compute data_head % data_size to find the location inside the perf event data buffer. The aux_head of a perf event aux buffer wraps around and always stays within the perf event aux buffer. Well, at least that's the behaviour for BTS and PT - where BTS uses the data buffer and PT the aux buffer. GDB does not read beyond data_head or aux_head. This is OK for BTS but wrong for PT. It causes only a portion of the trace to be considered by GDB. In the extreme case, the buffer may appear (almost) empty. Thanks to Tim Wiederhake <tim.wiederhake@intel.com> for reporting the anomaly. Change it to read the entire aux buffer for PT. The buffer is initially zero so any extra zeroes we read before aux_head wraps around the first time will be ignored when searching for the first PSB packet in order to synchronize onto the trace stream. gdb/ * nat/linux-btrace.c (perf_event_read): Allow data_head < size. * nat/linux-btrace.c (perf_event_read_all): Do not adjust size. Change-Id: If4f8049a2080a5f16f336309450b32a3eb1e3ec9
Diffstat (limited to 'gdb')
-rw-r--r--gdb/ChangeLog5
-rw-r--r--gdb/nat/linux-btrace.c19
2 files changed, 20 insertions, 4 deletions
diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b4398f6..600925f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2016-11-14 Markus Metzger <markus.t.metzger@intel.com>
+
+ * nat/linux-btrace.c (perf_event_read): Allow data_head < size.
+ * nat/linux-btrace.c (perf_event_read_all): Do not adjust size.
+
2016-11-12 Tom Tromey <tom@tromey.com>
* rust-exp.y (super_name): Use std::vector.
diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 5ac5db2..e4144c6 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -119,10 +119,24 @@ perf_event_read (const struct perf_event_buffer *pev, __u64 data_head,
if (size == 0)
return NULL;
+ /* We should never ask for more data than the buffer can hold. */
+ buffer_size = pev->size;
+ gdb_assert (size <= buffer_size);
+
+ /* If we ask for more data than we seem to have, we wrap around and read
+ data from the end of the buffer. This is already handled by the %
+ BUFFER_SIZE operation, below. Here, we just need to make sure that we
+ don't underflow.
+
+ Note that this is perfectly OK for perf event buffers where data_head
+ doesn'grow indefinitely and instead wraps around to remain within the
+ buffer's boundaries. */
+ if (data_head < size)
+ data_head += buffer_size;
+
gdb_assert (size <= data_head);
data_tail = data_head - size;
- buffer_size = pev->size;
begin = pev->mem;
start = begin + data_tail % buffer_size;
stop = begin + data_head % buffer_size;
@@ -153,10 +167,7 @@ perf_event_read_all (struct perf_event_buffer *pev, gdb_byte **data,
__u64 data_head;
data_head = *pev->data_head;
-
size = pev->size;
- if (data_head < size)
- size = (size_t) data_head;
*data = perf_event_read (pev, data_head, size);
*psize = size;