Fix 'tools/statsnoop' from failing to attach kprobes

This fixes 'tools/statsnoop' from failing to attach probes
when the expected entry point for a system call cannot be
found. This script uses the 'stat', 'statfs' and 'newstat'
system calls, all of which must be implemented to be POSIX
compliant. However, the names of the actual entry points
for their respective implementations in the kernel might
vary across architectures. For example, a powerpc64 kernel
does not define 'sys_stat' but still provides the 'stat'
system call via 'sys_newstat'. This causes the script to
fail if it tries to attach a probe at 'sys_stat'. We avoid
this by performing some extra checks to see if these entry
points exist.
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>

tools/statsnoop.py

@@ -105,12 +105,22 @@ if debug:
 
 # initialize BPF
 b = BPF(text=bpf_text)
-b.attach_kprobe(event="sys_stat", fn_name="trace_entry")
-b.attach_kprobe(event="sys_statfs", fn_name="trace_entry")
-b.attach_kprobe(event="sys_newstat", fn_name="trace_entry")
-b.attach_kretprobe(event="sys_stat", fn_name="trace_return")
-b.attach_kretprobe(event="sys_statfs", fn_name="trace_return")
-b.attach_kretprobe(event="sys_newstat", fn_name="trace_return")
+
+# for POSIX compliance, all architectures implement these
+# system calls but the name of the actual entry point may
+# be different for which we must check if the entry points
+# actually exist before attaching the probes
+if BPF.ksymname("sys_stat") != -1:
+    b.attach_kprobe(event="sys_stat", fn_name="trace_entry")
+    b.attach_kretprobe(event="sys_stat", fn_name="trace_return")
+
+if BPF.ksymname("sys_statfs") != -1:
+    b.attach_kprobe(event="sys_statfs", fn_name="trace_entry")
+    b.attach_kretprobe(event="sys_statfs", fn_name="trace_return")
+
+if BPF.ksymname("sys_newstat") != -1:
+    b.attach_kprobe(event="sys_newstat", fn_name="trace_entry")
+    b.attach_kretprobe(event="sys_newstat", fn_name="trace_return")
 
 TASK_COMM_LEN = 16    # linux/sched.h
 NAME_MAX = 255        # linux/limits.h