[PATCH 4/5] btrace: improve enable error messages

[Markus Metzger <markus dot t dot metzger at intel dot com>]

Improve the error message when GDB fails to start recording branch trace.

This patch also removes a zero buffer size check for PT to align with BTS.  The
buffer size can not be configured to be zero.

2018-01-25  Markus Metzger  <markus.t.metzger@intel.com>

gdb/
	* nat/linux-btrace.c (perf_event_pt_event_type): Improve error message.
	Return type.  Update callers.
	(linux_enable_bts, linux_enable_pt): Improve error message.
	(linux_enable_pt): Remove zero buffer size check.
	(linux_enable_btrace): Improve error messages.  Remove NULL return
	check.
---
 gdb/nat/linux-btrace.c | 69 +++++++++++++++++++++-----------------------------
 1 file changed, 29 insertions(+), 40 deletions(-)

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 08a9716..db93739 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -524,7 +524,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
   scoped_close_fd fd (syscall (SYS_perf_event_open, &bts->attr, pid,
 			       -1, -1, 0));
   if (fd < 0)
-    return nullptr;
+    error (_("Failed to start recording: %s"), safe_strerror (errno));
 
   /* Convert the requested size in bytes to pages (rounding up).  */
   pages = ((size_t) conf->size / PAGE_SIZE
@@ -560,6 +560,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
       if ((__u64) length != data_size + PAGE_SIZE)
 	continue;
 
+      errno = 0;
       /* The number of pages we request needs to be a power of two.  */
       header.reset (NULL, length, PROT_READ, MAP_SHARED, fd, 0);
       if (header != MAP_FAILED)
@@ -567,7 +568,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
     }
 
   if (pages == 0)
-    return nullptr;
+    error (_("Failed to map trace buffer: %s."), safe_strerror (errno));
 
   data_offset = PAGE_SIZE;
 
@@ -583,7 +584,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
 
       /* Check for overflows.  */
       if ((__u64) size != data_size)
-	return nullptr;
+	error (_("Failed to determine trace buffer size."));
     }
 #endif /* defined (PERF_ATTR_SIZE_VER5) */
 
@@ -600,21 +601,23 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
 
 #if defined (PERF_ATTR_SIZE_VER5)
 
-/* Determine the event type.
-   Returns zero on success and fills in TYPE; returns -1 otherwise.  */
+/* Determine the event type.  */
 
 static int
-perf_event_pt_event_type (int *type)
+perf_event_pt_event_type (void)
 {
-  gdb_file_up file =
-    gdb_fopen_cloexec ("/sys/bus/event_source/devices/intel_pt/type", "r");
+  const char *filename = "/sys/bus/event_source/devices/intel_pt/type";
+
+  errno = 0;
+  gdb_file_up file = gdb_fopen_cloexec (filename, "r");
   if (file.get () == nullptr)
-    return -1;
+    error (_("Failed to open %s: %s."), filename, safe_strerror (errno));
 
-  int found = fscanf (file.get (), "%d", type);
-  if (found == 1)
-    return 0;
-  return -1;
+  int type, found = fscanf (file.get (), "%d", &type);
+  if (found != 1)
+    error (_("Failed to read the PT event type from %s."), filename);
+
+  return type;
 }
 
 /* Enable branch tracing in Intel Processor Trace format.  */
@@ -624,14 +627,7 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
 {
   struct btrace_tinfo_pt *pt;
   size_t pages;
-  int pid, pg, errcode, type;
-
-  if (conf->size == 0)
-    return NULL;
-
-  errcode = perf_event_pt_event_type (&type);
-  if (errcode != 0)
-    return NULL;
+  int pid, pg;
 
   pid = ptid_get_lwp (ptid);
   if (pid == 0)
@@ -645,7 +641,7 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
   pt = &tinfo->variant.pt;
 
   pt->attr.size = sizeof (pt->attr);
-  pt->attr.type = type;
+  pt->attr.type = perf_event_pt_event_type ();
 
   pt->attr.exclude_kernel = 1;
   pt->attr.exclude_hv = 1;
@@ -654,14 +650,14 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
   errno = 0;
   scoped_close_fd fd (syscall (SYS_perf_event_open, &pt->attr, pid, -1, -1, 0));
   if (fd < 0)
-    return nullptr;
+    error (_("Failed to start recording: %s"), safe_strerror (errno));
 
   /* Allocate the configuration page. */
   scoped_mmap<perf_event_mmap_page> header (NULL, PAGE_SIZE,
 					    PROT_READ | PROT_WRITE, MAP_SHARED,
 					    fd, 0);
   if (header == MAP_FAILED)
-    return nullptr;
+    error (_("Failed to map trace user page: %s."), safe_strerror (errno));
 
   header->aux_offset = header->data_offset + header->data_size;
 
@@ -700,13 +696,14 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
 
       header->aux_size = data_size;
 
+      errno = 0;
       aux.reset (NULL, length, PROT_READ, MAP_SHARED, fd, header->aux_offset);
       if (aux != MAP_FAILED)
 	break;
     }
 
   if (pages == 0)
-    return nullptr;
+    error (_("Failed to map trace buffer: %s."), safe_strerror (errno));
 
   pt->pt.size = aux.size ();
   pt->pt.mem = aux.release ();
@@ -723,8 +720,7 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
 static struct btrace_target_info *
 linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
 {
-  errno = EOPNOTSUPP;
-  return NULL;
+  error (_("GDB does not support Intel PT."));
 }
 
 #endif /* !defined (PERF_ATTR_SIZE_VER5) */
@@ -734,27 +730,20 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
 struct btrace_target_info *
 linux_enable_btrace (ptid_t ptid, const struct btrace_config *conf)
 {
-  struct btrace_target_info *tinfo;
-
-  tinfo = NULL;
   switch (conf->format)
     {
     case BTRACE_FORMAT_NONE:
-      break;
+      error (_("Bad branch trace format."));
+
+    default:
+      error (_("Unknown branch trace format."));
 
     case BTRACE_FORMAT_BTS:
-      tinfo = linux_enable_bts (ptid, &conf->bts);
-      break;
+      return linux_enable_bts (ptid, &conf->bts);
 
     case BTRACE_FORMAT_PT:
-      tinfo = linux_enable_pt (ptid, &conf->pt);
-      break;
+      return linux_enable_pt (ptid, &conf->pt);
     }
-
-  if (tinfo == NULL)
-    error (_("Unknown error."));
-
-  return tinfo;
 }
 
 /* Disable BTS tracing.  */
-- 
1.8.3.1