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btrace: check perf_event_paranoid

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One recurring error on Debian systems is that the default perf_event_paranoid
setting disables the perf_event interface for user-space.

Check the current level and point the user to the file.

gdb/
	* nat/linux-btrace.c (diagnose_perf_event_open_fail): New.
	(linux_enable_pt, linux_enable_bts): Call
	diagnose_perf_event_open_fail.
This commit is contained in:
Markus Metzger 2018-01-19 15:17:43 +01:00
parent 17ad2a4f46
commit 88711fbfea
2 changed files with 36 additions and 2 deletions
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6
gdb/ChangeLog
View File

@ -1,3 +1,9 @@
2018-02-09 Markus Metzger <markus.t.metzger@intel.com>
* nat/linux-btrace.c (diagnose_perf_event_open_fail): New.
(linux_enable_pt, linux_enable_bts): Call
diagnose_perf_event_open_fail.
2018-02-09 Markus Metzger <markus.t.metzger@intel.com>
* nat/linux-btrace.c (perf_event_pt_event_type): Improve error message.

32
gdb/nat/linux-btrace.c
View File

@ -409,6 +409,34 @@ cpu_supports_bts (void)
}
}
/* The perf_event_open syscall failed. Try to print a helpful error
message. */
static void
diagnose_perf_event_open_fail ()
{
switch (errno)
{
case EPERM:
case EACCES:
{
static const char filename[] = "/proc/sys/kernel/perf_event_paranoid";
gdb_file_up file = gdb_fopen_cloexec (filename, "r");
if (file.get () == nullptr)
break;
int level, found = fscanf (file.get (), "%d", &level);
if (found == 1 && level > 2)
error (_("You do not have permission to record the process. "
"Try setting %s to 2 or less."), filename);
}
break;
}
error (_("Failed to start recording: %s"), safe_strerror (errno));
}
/* Enable branch tracing in BTS format. */
static struct btrace_target_info *
@ -448,7 +476,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
errno = 0;
scoped_fd fd (syscall (SYS_perf_event_open, &bts->attr, pid, -1, -1, 0));
if (fd.get () < 0)
error (_("Failed to start recording: %s"), safe_strerror (errno));
diagnose_perf_event_open_fail ();
/* Convert the requested size in bytes to pages (rounding up). */
pages = ((size_t) conf->size / PAGE_SIZE
@ -578,7 +606,7 @@ linux_enable_pt (ptid_t ptid, const struct btrace_config_pt *conf)
errno = 0;
scoped_fd fd (syscall (SYS_perf_event_open, &pt->attr, pid, -1, -1, 0));
if (fd.get () < 0)
error (_("Failed to start recording: %s"), safe_strerror (errno));
diagnose_perf_event_open_fail ();
/* Allocate the configuration page. */
scoped_mmap data (nullptr, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED,