Bug 419562

Summary:	PR_SET_PTRACER error with Ubuntu on WSL
Product:	[Developer tools] valgrind	Reporter:	Evan Hunter <evan>
Component:	general	Assignee:	Julian Seward <jseward>
Status:	REPORTED ---
Severity:	normal	CC:	leannap, philippe.waroquiers, pjfloyd
Priority:	NOR
Version First Reported In:	unspecified
Target Milestone:	---
Platform:	Microsoft Windows
OS:	Linux
Latest Commit:		Version Fixed In:
Sentry Crash Report:
Attachments:	Possible fix by falling back to alternate PR_SET_PTRACER call

Description Evan Hunter 2020-04-02 22:50:42 UTC

Created attachment 127224 [details]
Possible fix by falling back to alternate PR_SET_PTRACER call

SUMMARY
When running Ubuntu 18.04 on Windows-Subsystem-for-Linux (WSL), 
I have noticed the following error during every run of Valgrind:

error calling PR_SET_PTRACER, vgdb might block

STEPS TO REPRODUCE
1. Use Ubuntu 18.04 on WSL 
2. Build Valgrind from git 
   ./autogen.sh && ./configure && make -j8 --quiet check
3. Run valgrind against any program:
   VALGRIND_LIB=./.in_place VALGRIND_LIB_INNER=./.in_place/ ./coregrind/valgrind pwd

OBSERVED RESULT
  error calling PR_SET_PTRACER, vgdb might block

EXPECTED RESULT
  no error

SOFTWARE/OS VERSIONS
Windows: 10 (1909) 10.0.18363.720
Linux: Ubuntu 18.04 on WSL 
Valgrind: 3.16.0.GIT

ADDITIONAL INFORMATION

This is caused by a bug in noted in the following stack overflow: 
  https://stackoverflow.com/questions/57206233/valgrind-showing-error-calling-pr-set-ptracer-vgdb-might-block

It was logged with Microsoft as:
  https://github.com/microsoft/WSL/issues/3053

It has apparently been fixed in WSL build 17723: 
  https://docs.microsoft.com/en-us/windows/wsl/release-notes#build-17723-fast  

However it seems that it has not yet reached Windows Update (despite my PC being at build 18363)

The error comes from VG_(set_ptracer)() in coregrind/m_gdbserver/remote-utils.c
It appears that the following call fails:
    ret = VG_(prctl) (PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0);
But if the following alternate line is enabled, the call succeeds:
    ret = VG_(prctl) (PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0);

I believe we could work around this by just failing-over from the first prctl to the second. 
I will submit a patch which does that.

Comment 1 Philippe Waroquiers 2020-04-04 06:51:58 UTC

Thanks for the proposed patch.

I do not remember the reason for the line
   ret = VG_(prctl) (PR_SET_PTRACER, 1, 0, 0, 0);

prctl PR_SET_PTRACER documentation indicates that the second argument
is either PR_SET_PTRACER_ANY (to allow any process to ptrace the caller),
or a pid (to allow pid to ptrace the caller)
or 0 (to not allow anymore a process to ptrace the caller).

So, the reason of the call with the pid 1 is not clear (anymore to me.
I must have had a good reason at a time, but not commented :(.

That being said:
Does calling set_ptracer with value 1 effectively allow vgdb to
get a blocked valgrind process out of the syscall ?

In other words, before your patch:
   valgrind sleep 100
in another window:   vgdb help
and vgdb should block or give error msg or similar
     (you might use vgdb -d -d -d -d help   to get more info about what
      is going on)

and after your patch, vgdb -d -d -d -d help  should be able to wake up
valgrind and produce the help text.

Comment 2 Evan Hunter 2020-04-07 10:21:07 UTC

Thanks for the pointers on testing it on vgdb.
It looks like it still hangs vgdb :-(
I too am not sure what the prctl(PR_SET_PTRACER, 1, 0, 0, 0) call is trying to achieve. It seems to succeed but still leaves vgdb blocked.

Comment 3 Philippe Waroquiers 2020-04-13 10:37:14 UTC

(In reply to Evan Hunter from comment #2)
> Thanks for the pointers on testing it on vgdb.
> It looks like it still hangs vgdb :-(
> I too am not sure what the prctl(PR_SET_PTRACER, 1, 0, 0, 0) call is trying
> to achieve. It seems to succeed but still leaves vgdb blocked.

Using the debug options -d -d -d of vgdb and -v -v -v -d -d -d of valgrind,
it should be possible to have an idea about what does not work.

Comment 4 Tom Hughes 2021-10-07 08:41:54 UTC

*** Bug 443426 has been marked as a duplicate of this bug. ***

Comment 5 Paul Floyd 2025-04-23 15:36:05 UTC

I don't see any problem with WSL2 on Windows 11, openSUSE-Leap-15.6