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Bug 225583 - UML on FC6 fails because of utrace patch
Summary: UML on FC6 fails because of utrace patch
Alias: None
Product: Fedora
Classification: Fedora
Component: kernel
Version: 6
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Roland McGrath
QA Contact: Brian Brock
Depends On:
TreeView+ depends on / blocked
Reported: 2007-01-31 14:51 UTC by JuanJo Ciarlante
Modified: 2007-11-30 22:11 UTC (History)
3 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2007-09-04 20:45:23 UTC

Attachments (Terms of Use)

Description JuanJo Ciarlante 2007-01-31 14:51:05 UTC
Description of problem:
Running any user-mode-linux kernel (did try self compiled 2.6.17 and dloaded
2.6.18, umls) fails: it boots and stops responding after 
"VFS: Mounted root (ext3 filesystem) readonly."

I tracked back and narrowed the kernel versions where the problem
started to show:
   OK:  kernel-smp-2.6.17-1.2187_FC5 (last FC5 upd. kernel I know to run ok)
   BAD: kernel-smp-2.6.18-1.2239.fc5 (IIRC 1st non-working FC5 kernel)
   BAD: kernel-2.6.19-1.2895.fc6     (current FC6 kernel)

Note: this is a FC6 uptodate distro, booted with these 3 diff. kernels.
Because those FC5 kernels have same selinux policyversion , FC6 "environment"
runs ok AFAICT.

To see at least if the command gets "enough" CPU to halt the UML, I tried:     
 ./linux init=/sbin/halt -f                                                    
It didn't, "ps lx" on the host shows:                                          
   $ ps lx                                                                     
   0 240 4284 4152 15 0 33000 11000 - R+ pts/1 3:31 ./linux (Jj9kkn) /sbin/halt]
   1 240 4293 4284 25 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4294 4284 24 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4295 4284 15 0 33000 11000 - S+ pts/1 0:00 ./linux (Jj9kkn) /sbin/halt]
   1 240 4296 4284 15 0 60 56 -  R+ pts/1 2:42 [linux]                         
, where cpu time for those "R+" procs almost equals wall time;                 
strace'ing repeats eternally these lines:                                      
   $ strace -p 4284                                                            
   -- SIGCHLD (Child exited) @ 0 (0) ---                                       
   waitpid(4296, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSEGV}], WSTOPPED) = 4296 
   ptrace(PTRACE_GETREGS, 4296, 0, 0xa06ab82c) = 0                             
   ptrace(PTRACE_GETFPXREGS, 4296, 0, 0xa06ab8dc) = 0                          
   ptrace(PTRACE_CONT, 4296, 0, SIGSEGV) = 0                                   

Version-Release number of selected component (if applicable):

How reproducible:
Check that you are running a stock FC6 kernel
Compile or download an UML guest kernel (eg:
and any root_fs image, boot UML: it will stop after VFS mount.

Steps to Reproduce:
1. Download/compile UML guest kernel, eg: 
   set -e
   mv kernel32- linux
   chmod +x linux

2. Download/build UML guest root_fs, eg: 
   bunzip2 DSL-2.2-root_fs.bz2
   mv DSL-2.2-root_fs root_fs

3. ./linux
Actual results:
UML stops actually running after:
"VFS: Mounted root (ext3 filesystem) readonly."

Expected results:
To continue normal guest boot (run /sbin/init)

Additional info:
I've posted the FULL report uml-users mailing list:

Comment 1 JuanJo Ciarlante 2007-01-31 14:53:05 UTC
I forgot metioning I did more testing:

I did the following tests, based on released utrace patch (OK means UML runs
w/no problem, BAD means UML stops after VFS root mounting )
#1 vanilla: OK
#2,CONFIG_{UP}TRACE=y    : BAD (same behaviour as FC6 stock kernel)
#3,CONFIG_{UP}TRACE unset: BAD (idem)

Curiously, #3 did the very same as #2 (even wrt strace process disposition
and strace "behaviour"), I'd expected #3 to simply fail at boot with
EINVAL or alike (given that no ptrace support was compiled in).


Comment 2 Roland McGrath 2007-02-06 05:12:14 UTC
Would it be possible for you to test against vanilla 2.6.20?
The utrace patches in are now
against 2.6.20, and have some fixes not in the 2.6.18 backport.  If utrace is
breaking UML, I'll need to debug it against the current kernel.

Comment 3 JuanJo Ciarlante 2007-02-06 20:22:05 UTC
I've tested against 2.6.20: SAME (bad) behaviour as

FYI ss with previous kernels, I've downloaded  2.6.20 vanilla, copied .config
make && make install && boot ; later applied utrace patch , edited EXTRA_VERSION
&& ... && boot.

Comment 4 Mark Wielaard 2007-03-08 22:50:32 UTC
According to Jeff Dike "UML runs on the utrace in -mm (i.e. utrace on the host),
which it didn't with several Fedora kernels."

Comment 5 Roland McGrath 2007-03-08 22:52:56 UTC
Indeed, apparently problems with UML are now seen only on a CONFIG_PREEMPT=y
kernel, which Fedora does not ship.  However, it may be only the luck of the
timing, as the bug biting UML was never isolated.

Comment 6 Roland McGrath 2007-09-04 20:45:23 UTC
Several old bugs that might have affected this have been fixed.  Reopen if the
problem reoccurs.

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