aya: fix pid=0 handling in uprobe attach

perf_event_open treats pid=0 as the calling process/thread, so
Some(0) should carry that meaning in UProbe::attach. However,
aya unconditionally reads /proc/<pid>/maps when pid is Some, but
/proc/0/maps does not exist.

Use the current pid only for the ProcMap lookup, and keep pid=0 for
the actual attach semantics. Add documentation and an integration test.

Author: swananan

aya/src/programs/uprobe.rs

@@ -109,7 +109,8 @@ impl UProbe {
     /// Attaches the uprobe to the function `fn_name` defined in the `target`.
     /// If the attach point specifies an offset, it is added to the address of
     /// the target function. If `pid` is not `None`, the program executes only
-    /// when the target function is executed by the given `pid`.
+    /// when the target function is executed by the given `pid`; `Some(0)`
+    /// means the calling process/thread.
     ///
     /// The `target` argument can be an absolute path to a binary or library, or
     /// a library name (eg: `"libc"`).
@@ -131,7 +132,10 @@ impl UProbe {
         pid: Option<u32>,
     ) -> Result<UProbeLinkId, ProgramError> {
         let UProbeAttachPoint { location, cookie } = point.into();
-        let proc_map = pid.map(ProcMap::new).transpose()?;
+        // /proc/0/maps does not exist; use the current pid only for ProcMap
+        // resolution and preserve pid=0 for the actual attach semantics.
+        let proc_map_pid = pid.map(|pid| if pid == 0 { std::process::id() } else { pid });
+        let proc_map = proc_map_pid.map(ProcMap::new).transpose()?;
         let path = resolve_attach_path(target.as_ref(), proc_map.as_ref())?;
         let (symbol, offset) = match location {
             UProbeAttachLocation::Symbol(s) => (Some(s), 0),

test/integration-test/src/tests/load.rs

@@ -368,6 +368,23 @@ fn basic_uprobe() {
     );
 }
 
+#[test_log::test]
+fn basic_uprobe_pid_zero_is_self() {
+    type P = UProbe;
+
+    let program_name = "test_uprobe";
+    let attach = |prog: &mut P| {
+        prog.attach("uprobe_function", "/proc/self/exe", Some(0))
+            .unwrap()
+    };
+    run_unload_program_test(
+        crate::TEST,
+        program_name,
+        attach,
+        aya::features().bpf_perf_link(), // probe uses perf_attach.
+    );
+}
+
 #[test_log::test]
 fn basic_flow_dissector() {
     type P = FlowDissector;