Failed to debug builtins with gdb or lldb

[Jiading Guo]

Hi all,

I'm following [1] to debug builtins (specifically `InterpreterEntryTrampoline`) with gdb. But it segfaults when I step through the code. After checking the assembly it turns out that the instruction has been modified.

The desired instructions provided by `--print-builtin-code` are as follows:

kind = BUILTIN

name = InterpreterEntryTrampoline

compiler = unknown

address = 0x171700005181

...

Instructions (size = 1584)

0x171707687180   0 448b570b       movl r10,[rdi+0xb]   

0x171707687184   4 4d03d6        REX.W addq r10,r14

...

but as shown in gdb, %r10 is replaced with %edx in the first instruction:

0x223e07687181   mov  0xb(%rdi),%edx

0x223e07687184   add  %r14,%r10

...

When I run the program without setting breakpoint on `Builtins_InterpreterEntryTrampoline` in gdb, it exits normally. [1] mentioned that "GDB modifies the binary". I'm wondering if gdb is modifying the memory content here? How can I prevent this?

I also tried lldb, but after setting the breakpoint on `Builtins_InterpreterEntryTrampoline`, it didn't stop at this function.

I'm using Ubuntu 20.04 with gdb 9.2 and lldb 10.0. Any help would be much appreciated.

Regards,

Jiading

[1] https://v8.dev/docs/gdb

[2] args.gn:

is_component_build = true

is_debug = true

symbol_level = 2

target_cpu = "x64"

use_goma = false

v8_enable_backtrace = true

v8_enable_fast_mksnapshot = true

v8_enable_slow_dchecks = true

v8_optimized_debug = false

```

[3] foo.js:

function foo() { return 1; }

foo();

%OptimizeFunctionOnNextCall(foo);

foo();

foo();

[Jakob Gruber]

gdb does modify the code when a breakpoint is set. My guess is, builtin-unembedding for short builtin calls copies the modified code into the v8 heap. You could try to disable short builtin calls. https://source.chromium.org/chromium/chromium/src/+/main:v8/BUILD.gn;l=197;drc=38f97cae6bc93330b8ad916a3faefb8ec35352e1

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[Jiading Guo]

Turning off builtin-unembedding helps!

I'm curious about how gdb modify the code when a breakpoint is set. I couldn't find useful links about this after searching. Could you provide some hint? Thank you!

[Jakob Gruber]

Searching for "how gdb breakpoints work" seems to have a few relevant results :)

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