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author | Denys Vlasenko <vda.linux@googlemail.com> | 2016-10-27 23:51:19 +0200 |
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committer | Denys Vlasenko <vda.linux@googlemail.com> | 2016-10-27 23:51:19 +0200 |
commit | 458c1f218bb6a6bd0325f0b0cedea5ab0980dbc3 (patch) | |
tree | bb5383793b49f8e79b605751c88a7d7b767acd2d /shell/hush_test/hush-parsing/escape4.right | |
parent | c0663c7cd218e23a9c256491c787203a07efb666 (diff) | |
download | busybox-458c1f218bb6a6bd0325f0b0cedea5ab0980dbc3.tar.gz |
ash: [JOBS] Fix dowait signal race
Upstream commit:
Date: Sun, 22 Feb 2009 18:10:01 +0800
[JOBS] Fix dowait signal race
This test program by Alexey Gladkov can cause dash to enter an
infinite loop in waitcmd.
#!/bin/dash
trap "echo TRAP" USR1
stub() {
echo ">>> STUB $1" >&2
sleep $1
echo "<<< STUB $1" >&2
kill -USR1 $$
}
stub 3 &
stub 2 &
until { echo "###"; wait; } do
echo "*** $?"
done
The problem is that if we get a signal after the wait3 system
call has returned but before we get to INTON in dowait, then
we can jump back up to the top and lose the exit status. So
if we then wait for the job that has just exited, then it'll
stay there forever.
I made the original change that caused this bug to fix pretty
much the same bug but in the opposite direction. That is, if
we get a signal after we enter wait3 but before we hit the kernel
then it too can cause the wait to go on forever (assuming the
child doesn't exit).
In fact this is pretty much exactly the scenario that you'll
find in glibc's documentation on pause(). The solution is given
there too, in the form of sigsuspend, which is the only way to
do the check and wait atomically.
So this patch fixes Alexey's race without reintroducing the old
bug by converting the blocking wait3 to a sigsuspend.
In order to do this we need to set a signal handler for SIGCHLD,
so the code has been modified to always do that.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
I failed to reproduce the bug (it requires precise timing), but it seems real.
function old new delta
dowait 284 463 +179
setsignal 301 326 +25
signal_handler 59 76 +17
ash_main 1481 1487 +6
localcmd 350 348 -2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/1 up/down: 227/-2) Total: 225 bytes
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Diffstat (limited to 'shell/hush_test/hush-parsing/escape4.right')
0 files changed, 0 insertions, 0 deletions