gdb: attach to process

[Thiago Farina]

Hi,

I'm trying to use gdb to attach to a renderer process, and I would like how others have been doing as well.

So far I did:

In one terminal window:

$ ninja -C out/Debug content_shell

$ out/Debug/content_shell --no-sandbox --renderer-startup-dialog

# The output was something like:

# 22824:22824:0306/193347:1124504976507:ERROR:child_process.cc(132)] Renderer (22824) paused waiting for debugger to attach. Send SIGUSR1 to unpause.

In another terminal window:

$ gdb out/Debug/content_shell 22824

# then gdb starts throwing a bunch of "Reading symbols from ..."

# until it stops at

(gdb)

After that what should I do?

Btw, I also tried lldb, but it is unusable on Linux.

In lldb what I got was:

$ lldb

(lldb) process attach --pid 22824

Process 22824 stopped

* thread #1: tid = 22824, 0x00007fbbd63b75d0 libpthread.so.0`__libc_pause + 16, name = 'content_shell', stop reason = trace

    frame #0: 0x00007fbbd63b75d0 libpthread.so.0`__libc_pause + 16

libpthread.so.0`__libc_pause:

->  0x7fbbd63b75d0 <+16>: cmpq   $-0xfff, %rax

    0x7fbbd63b75d6 <+22>: jae    0x7fbbd63b7609            ; <+73>

    0x7fbbd63b75d8 <+24>: retq   

libpthread.so.0`???:

    0x7fbbd63b75d9 <+25>: subq   $0x8, %rsp

Executable module set to "/home/tfarina/chromium/src/out/Debug/content_shell".

Architecture set to: x86_64-unknown-linux-unknown.

-- 

Thiago Farina

[Daniel Cheng]

Set your breakpoints and what not in gdb, then use signal SIGUSR1 in gdb to resume execution.

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[Nico Weber]

Hit 'c', then enter. (Or set breakpoints first, and then hit 'c'.)

There are gdb tutorials available on an internet of your choice.

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[Chris Harrelson]

I just use --single-process and run the binary directly within gdb. Does that work for your use case?

Chris

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[Thiago Farina]

On Fri, Mar 6, 2015 at 8:07 PM, Chris Harrelson <chri...@chromium.org> wrote:

I just use --single-process and run the binary directly within gdb. Does that work for your use case?

That does not seem to be recommended by http://www.chromium.org/developers/how-tos/debugging-on-os-x#TOC-a-Run-Chrome-in-a-single-process

Though that does not seem to be documented the same at http://www.chromium.org/developers/how-tos/debugging-on-windows#TOC-Single-process-mode and in https://code.google.com/p/chromium/wiki/LinuxDebugging#Single-Process_mode

So I want to debug using something that is actually tested and recommended and thus --renderer-startup-dialog seems to be the way to go.

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Thiago Farina

[Chris Harrelson]

But if it works it's simpler, so it's what I try first. :)

Also, it doesn't really work very well for out/Debug/chrome, just out/Debug/content_shell. My go-to commandline is

cgdb --args ./out/Debug/content_shell --single-process

[Thiago Farina]

On Fri, Mar 6, 2015 at 7:54 PM, Daniel Cheng <dch...@chromium.org> wrote:

Set your breakpoints and what not in gdb, then use signal SIGUSR1 in gdb to resume execution.

 Thanks Daniel!

Doing that worked.

(gdb) b content/renderer/render_thread_impl.cc:1002

Breakpoint 1 at 0x7fbbe0273590: file ../../content/renderer/render_thread_impl.cc, line 1002.

(gdb) signal SIGUSR1

Continuing with signal SIGUSR1.

[New Thread 0x7fbbcd5d1700 (LWP 26755)]

[New Thread 0x7fbbccdd0700 (LWP 26763)]

Breakpoint 1, content::RenderThreadImpl::EnsureWebKitInitialized (

    this=0x1522b376bc20)

    at ../../content/renderer/render_thread_impl.cc:1002

1002  blink::initialize(blink_platform_impl_.get());

-- 

Thiago Farina

[Nasko Oskov]

I personally use --renderer-cmd-prefix, which starts a new terminal for each renderer I want to debug. To make it more manageable, I'd suggest reading a couple of sections of the Linux debugging document:

* Choosing which renderers to debug - let's you quickly bail out if the specific renderer is of no interest to be debugged

* Selective breakpoints - let's you automate lots of the manual tasks in getting to the point of interest.

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[Stefan Zager]

+1 for --renderer-cmd-prefix, that does the trick for me.

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[Haojian Wu]

On Sat, Mar 7, 2015 at 7:20 AM, Thiago Farina <tfa...@chromium.org> wrote:

(gdb) b content/renderer/render_thread_impl.cc:1002

Breakpoint 1 at 0x7fbbe0273590: file ../../content/renderer/render_thread_impl.cc, line 1002.

BTW, is there any more convenient way to set breakpoint? Typing the whole file path for setting each breakpoint is quite troublesome.

[Jeremy Roman]

You should only need the basename; since there is no other render_frame_impl.cc, it should do the right thing.

FWIW, I usually don't care about process startup, so I generally start with "out/Debug/chrome --no-sandbox --disable-hang-monitor", grab the renderer PID from the task manager, then use "gdb [pid]", set breakpoints, and continue.

As another option, Steve Kobes recently posted another way to debug a renderer (with a script landed in Blink): https://groups.google.com/a/chromium.org/d/msg/blink-dev/XyE1DeqMPpg/Bao3H2_bLi4J

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[Thiago Farina]

On Sat, Mar 7, 2015 at 1:03 AM, Jeremy Roman <jbr...@chromium.org> wrote:

You should only need the basename; since there is no other render_frame_impl.cc, it should do the right thing.

FWIW, I usually don't care about process startup, so I generally start with "out/Debug/chrome --no-sandbox --disable-hang-monitor", grab the renderer PID from the task manager, then use "gdb [pid]", set breakpoints, and continue.

As another option, Steve Kobes recently posted another way to debug a renderer (with a script landed in Blink): https://groups.google.com/a/chromium.org/d/msg/blink-dev/XyE1DeqMPpg/Bao3H2_bLi4J

Thanks Jeremy. Steve's script does exactly what I was trying to do manually (with the upside of having to open two terminal windows).

 

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Thiago Farina