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: b...@freebsd.org
Reporter: corydo...@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.
_______________________________________________
freebsd-bugs@freebsd.org mailing list
https://lists.freebsd.org/mailman/listinfo/freebsd-bugs
To unsubscribe, send any mail to "freebsd-bugs-unsubscr...@freebsd.org"
