codegen: Fix segfault of CodegenLLVM::generateProbe()

If it is dummy, then the visit() operation should not be executed. For
example, on aarch64, there is no syscalls:sys_{enter,exit}_open
tracepoint, however, opensnoop.bt tool will try to attch them and visit
args's field. We expect to get a warning, but instead we get a segmentation
fault. As follows:

    $ sudo gdb bpftrace
    ...
    (gdb) set args opensnoop.bt
    (gdb) r
    opensnoop.bt:22-24: WARNING: tracepoint not found: syscalls:sys_enter_open
    opensnoop.bt:28-30: WARNING: tracepoint not found: syscalls:sys_exit_open

    Thread 1 "bpftrace" received signal SIGSEGV, Segmentation fault.
    0x000000000052bbd8 in __gnu_cxx::__normal_iterator<bpftrace::Field const*, std::vector<bpftrace::Field, std::allocator<bpftrace::Field> > >::__normal_iterator (this=0xffffffffb0b8, __i=<error reading variable: Cannot access memory at address 0x10>)
        at /usr/include/c++/14/bits/stl_iterator.h:1068
    1068              : _M_current(__i) { }
    (gdb) bt
    #0  0x000000000052bbd8 in __gnu_cxx::__normal_iterator<bpftrace::Field const*, std::vector<bpftrace::Field, std::allocator<bpftrace::Field> > >::__normal_iterator (this=0xffffffffb0b8,
        __i=<error reading variable: Cannot access memory at address 0x10>) at /usr/include/c++/14/bits/stl_iterator.h:1068
    bpftrace#1  0x0000000000525bdc in std::vector<bpftrace::Field, std::allocator<bpftrace::Field> >::begin (this=0x10)
        at /usr/include/c++/14/bits/stl_vector.h:884
    bpftrace#2  0x000000000059ce8c in bpftrace::Struct::GetField (this=0x0, name="filename")
        at /home/rongtao/Git/bpftrace/bpftrace/src/struct.cpp:131
    bpftrace#3  0x00000000005a70a0 in bpftrace::SizedType::GetField (this=0xe9b998, name="filename")
        at /home/rongtao/Git/bpftrace/bpftrace/src/types.cpp:538
    bpftrace#4  0x00000000007c7178 in bpftrace::ast::CodegenLLVM::visit (this=0xffffffffc650, acc=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:1973
    bpftrace#5  0x0000000000847db0 in bpftrace::ast::FieldAccess::accept (this=0xe9be30, v=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/ast.cpp:31
    bpftrace#6  0x00000000007d2404 in bpftrace::ast::CodegenLLVM::accept (this=0xffffffffc650, node=0xe9be30)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:3778
    bpftrace#7  0x00000000007c8bec in bpftrace::ast::CodegenLLVM::visit (this=0xffffffffc650, assignment=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:2218
    bpftrace#8  0x0000000000847eb4 in bpftrace::ast::AssignMapStatement::accept (this=0xffffe4429250, v=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/ast.cpp:36
    bpftrace#9  0x00000000007d2404 in bpftrace::ast::CodegenLLVM::accept (this=0xffffffffc650, node=0xffffe4429250)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:3778
    bpftrace#10 0x00000000007ca4c4 in bpftrace::ast::CodegenLLVM::generateProbe (this=0xffffffffc650, probe=..., full_func_id="dummy",
        name="dummy", func_type=0xff2b50, usdt_location_index=std::optional [no contained value], dummy=true)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:2539
    bpftrace#11 0x00000000007cb4e4 in bpftrace::ast::CodegenLLVM::visit (this=0xffffffffc650, probe=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:2734
    bpftrace#12 0x00000000008480f0 in bpftrace::ast::Probe::accept (this=0xffffe4427650, v=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/ast.cpp:47
    bpftrace#13 0x00000000007d2404 in bpftrace::ast::CodegenLLVM::accept (this=0xffffffffc650, node=0xffffe4427650)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:3778
    bpftrace#14 0x00000000007cb7a4 in bpftrace::ast::CodegenLLVM::visit (this=0xffffffffc650, program=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:2752
    bpftrace#15 0x000000000084818c in bpftrace::ast::Program::accept (this=0xe90f50, v=...)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/ast.cpp:50
    bpftrace#16 0x00000000007d2404 in bpftrace::ast::CodegenLLVM::accept (this=0xffffffffc650, node=0xe90f50)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:3778
    bpftrace#17 0x00000000007d03fc in bpftrace::ast::CodegenLLVM::generate_ir (this=0xffffffffc650)
        at /home/rongtao/Git/bpftrace/bpftrace/src/ast/passes/codegen_llvm.cpp:3450
    bpftrace#18 0x0000000000478138 in main (argc=2, argv=0xfffffffff4c8) at /home/rongtao/Git/bpftrace/bpftrace/src/main.cpp:898

We can simplify opensnoop.bt to:

    tracepoint:syscalls:sys_enter_open_not_exist,
    tracepoint:syscalls:sys_enter_openat
    {
        @ = args.filename;
    }

This will produce the following error:

    stdin:1:1-45: WARNING: tracepoint not found: syscalls:sys_enter_open_not_exist
    tracepoint:syscalls:sys_enter_open_not_exist,tracepoint:syscalls:sys_enter_openat {@ = args.filename;}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Segmentation fault

We need to generate the dummy probe only for cases when none of the probe
attach points exists.

Link: bpftrace#3274
Signed-off-by: Viktor Malik <[email protected]>
Signed-off-by: Rong Tao <[email protected]>

CHANGELOG.md

@@ -29,6 +29,8 @@ and this project adheres to
 - Remove the `-dd` CLI option
   - [#3203](https://github.com/bpftrace/bpftrace/pull/3203)
 #### Fixed
+- Fix segfault for multi-tracepoint probes
+  - [#3274](https://github.com/bpftrace/bpftrace/pull/3274)
 #### Security
 #### Docs
 - Remove mention of unsupported character literals

src/ast/passes/codegen_llvm.cpp

@@ -2710,6 +2710,7 @@ void CodegenLLVM::visit(Probe &probe)
   // We begin by saving state that gets changed by the codegen pass, so we
   // can restore it for the next pass (printf_id_, time_id_).
   auto reset_ids = async_ids_.create_reset_ids();
+  bool generated = false;
   for (auto *attach_point : *probe.attach_points) {
     reset_ids();
     current_attach_point_ = attach_point;
@@ -2739,16 +2740,18 @@ void CodegenLLVM::visit(Probe &probe)
 
         auto match_ap = attach_point->create_expansion_copy(match);
         add_probe(match_ap, probe, match, func_type);
-      }
-      if (matches.empty()) {
-        generateProbe(probe, "dummy", "dummy", func_type, std::nullopt, true);
+        generated = true;
       }
     } else {
       if (probe.index() == 0)
         probe.set_index(getNextIndexForProbe());
       add_probe(*attach_point, probe, attach_point->name(), func_type);
+      generated = true;
     }
   }
+  if (!generated) {
+    generateProbe(probe, "dummy", "dummy", func_type, std::nullopt, true);
+  }
 
   current_attach_point_ = nullptr;
 }