[Bug 246385] SIGCHLD dropped if generated while blocked in sigfastblock

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246385

            Bug ID: 246385
           Summary: SIGCHLD dropped if generated while blocked in
                    sigfastblock
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: corydoras at ridiculousfish.com
 Attachment #214394 text/plain
         mime type:

Created attachment 214394
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=214394&action=edit
reduced test case, see description for how to compile

fish-shell is observing a hang under FreeBSD CURRENT which I believe to be a
regression in FreeBSD signal handling, relative to 12.1.

Originally reported as https://github.com/fish-shell/fish-shell/issues/6919

High level: if a process forks and the child exits before fork is complete in
the parent, then SIGCHLD will not be delivered.

Details: fork() will block signals using sigfastblock. If the child dies before
signals are unblocked, the SIGCHLD will be marked as pending via the
sigfastblock word. In this case fork() will issue a syscall to
`sigfastblock(SIGFASTBLOCK_UNBLOCK)`; however SIGCHLD will NOT be delivered.

Reduced test case attached as `demo.c`. This test case uses a loop which
creates a child and waits for SIGCHLD to be delivered via the self-pipe trick.
It counts every 256 iterations.

To reproduce:

    clang demo.c ; ./a.out # this does not hang

    clang -lpthread demo.c ; ./a.out # this hangs on FreeBSD CURRENT only


ktrace output:

Good iteration (SIGCHLD generated while signals NOT blocked):
 27181 a.out    CALL  fork
 27181 a.out    RET   fork 27581/0x6bbd
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)
 27181 a.out    RET   read RESTART
 27181 a.out    PSIG  SIGCHLD caught handler=0x800258dd0 mask=0x0
code=CLD_EXITED
 27181 a.out    CALL  sigprocmask(SIG_SETMASK,0x7fffffffe24c,0)
 27181 a.out    RET   sigprocmask 0
 27181 a.out    CALL  write(0x4,0x7fffffffde6b,0x1)
 27181 a.out    GIO   fd 4 wrote 1 byte
 27181 a.out    RET   write 1
 27181 a.out    CALL  sigreturn(0x7fffffffde80)
 27181 a.out    RET   sigreturn JUSTRETURN
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)
 27181 a.out    GIO   fd 3 read 1 byte


Hanging iteration (SIGCHLD generated while signals ARE blocked):

 27181 a.out    RET   read 1
 27181 a.out    CALL  wait4(0x6bbd,0x7fffffffe8d4,0x6<WUNTRACED|WCONTINUED>,0)
 27181 a.out    RET   wait4 27581/0x6bbd
 27181 a.out    CALL  fork
 27181 a.out    RET   fork 27582/0x6bbe
 27181 a.out    CALL  sigfastblock(0x2,0)
 27181 a.out    RET   sigfastblock 0
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)

Note the call to sigfastblock(0x2) (pending signal) but PSIG  SIGCHLD is not
generated.

-- 
You are receiving this mail because:
You are the assignee for the bug.