Hello internals,
I'm trying to fix deadlock in an ancient php 5.2.17, php hangs on
internal libc lock.
Backtrace follows
#0 0x0000030b555024cb in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1 0x0000030b554986b8 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#2 0x0000030b55496aa1 in free () from /lib/x86_64-linux-gnu/libc.so.6
#3 0x0000000000734989 in php_error_cb (type=1,
error_filename=0x100013bbf18 "xxx", error_lineno=7, format=<optimized
out>,
args=<optimized out>) at main/main.c:837
#4 0x0000000000623e15 in soap_error_handler (error_num=1,
error_filename=0x100013bbf18 "/class.cbase.php", error_lineno=7,
format=0xa749c0 "Maximum execution time of %d second%s exceeded",
args=0x3a2de4d3fa0) at //ext/soap/soap.c:2115
#5 0x0000000000777066 in zend_error (type=1, format=0xa749c0 "Maximum
execution time of %d second%s exceeded") at /Zend/zend.c:976
#6 <signal handler called>
#7 0x0000030b55493302 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#8 0x0000030b55496aac in free () from /lib/x86_64-linux-gnu/libc.so.6
#9 0x0000030b57f817c5 in free_root () from
/usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
#10 0x0000030b57f65a34 in free_rows () from
/usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
#11 0x0000030b57f6620d in mysql_free_result () from
/usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
#12 0x000000000058cd1c in _free_mysql_result (rsrc=<optimized out>) at
ext/mysql/php_mysql.c:275
#13 0x000000000078340e in list_entry_destructor (ptr=0x5074760) at
Zend/zend_list.c:184
From my understanding free is not safe to use in a signal handler, and
this seems to be the issue here.
http://marc.info/?l=php-internals&m=121999390109071&w=2 seems to
address this issue but it's not present in 5.3 or later releases.
Very similar deadlock but with time() instead of free(),
https://bugs.php.net/bug.php?id=31749
Current php also uses free() in php_error_cb() and external
destructors in list_entry_destructor() aren't protected by
HANDLE_BLOCK_INTERRUPTIONS() (which seems to be a noop in fastcgi
mode), so I suspect 5.5 may also contain this deadlock.
main/main.c
php_error_cb()
835 if (display) {
836 if (PG(last_error_message)) {
837 free(PG(last_error_message));
... ^^^^^^ deadlock if previous free was
interrupted by a timeout signal
845 PG(last_error_type) = type;
846 PG(last_error_message) = strdup(buffer);
I'm thinking about "fixing" this by leaking memory pointed by
PG(last_error_message) if php called when a timeout is pending
(timeouts are usually very rare, php processes will eventually be
restarted so this little memory waste won't have time to make any
impact.
Is this issue fixed in modern php ? If so I would be grateful for some
information about the way it was done. This would save me a lot of
time trying to trigger
a non existing confition.
I will try to reproduce this in a modern version of php.
Thanks,
Michal Grzedzicki
Hello internals,
I'm trying to fix deadlock in an ancient php 5.2.17, php hangs on
internal libc lock.
From my understanding free is not safe to use in a signal handler, and
this seems to be the issue here.
No, it's not.
http://marc.info/?l=php-internals&m=121999390109071&w=2 seems to
address this issue but it's not present in 5.3 or later releases.Very similar deadlock but with
time()instead of free(),
https://bugs.php.net/bug.php?id=31749
Are you sure it's withtime()? I do see a free() in that call stack
(and no time),
as I would expect, astime()is required by POSIX.1-2004 to be
Async-signal-safe.
Current php also uses free() in php_error_cb() and external
destructors in list_entry_destructor() aren't protected by
HANDLE_BLOCK_INTERRUPTIONS() (which seems to be a noop in fastcgi
mode), so I suspect 5.5 may also contain this deadlock.main/main.c
php_error_cb()
835 if (display) {
836 if (PG(last_error_message)) {
837 free(PG(last_error_message));
... ^^^^^^ deadlock if previous free was
interrupted by a timeout signal
845 PG(last_error_type) = type;
846 PG(last_error_message) = strdup(buffer);I'm thinking about "fixing" this by leaking memory pointed by
PG(last_error_message) if php called when a timeout is pending
(timeouts are usually very rare, php processes will eventually be
restarted so this little memory waste won't have time to make any
impact.Is this issue fixed in modern php ? If so I would be grateful for some
information about the way it was done. This would save me a lot of
time trying to trigger
a non existing confition.I will try to reproduce this in a modern version of php.
It probably isn't. PG(last_error_message) is only modified in main.c, I
would try to use a separate arena for PG(last_error_message) and
PG(last_error_file). For instance it could be a static buffer reused
during the whole execution and extended with mmap(2) in the unlikely
case it turns out to be too small. I suspect it would also make the
error handler faster, as it would avoid the free() + malloc()
2013/9/18 Ángel González keisial@gmail.com:
Hello internals,
I'm trying to fix deadlock in an ancient php 5.2.17, php hangs on
internal libc lock.
From my understanding free is not safe to use in a signal handler, and
this seems to be the issue here.No, it's not.
http://marc.info/?l=php-internals&m=121999390109071&w=2 seems to
address this issue but it's not present in 5.3 or later releases.Very similar deadlock but with
time()instead of free(),
https://bugs.php.net/bug.php?id=31749Are you sure it's with
time()? I do see a free() in that call stack (and no
time),
as I would expect, astime()is required by POSIX.1-2004 to be
Async-signal-safe.
time() refers to https://bugs.php.net/bug.php?id=31749, You are right
this deadlock is related to free()
Current php also uses free() in php_error_cb() and external
destructors in list_entry_destructor() aren't protected by
HANDLE_BLOCK_INTERRUPTIONS() (which seems to be a noop in fastcgi
mode), so I suspect 5.5 may also contain this deadlock.main/main.c
php_error_cb()
835 if (display) {
836 if (PG(last_error_message)) {
837 free(PG(last_error_message));
... ^^^^^^ deadlock if previous free was
interrupted by a timeout signal
845 PG(last_error_type) = type;
846 PG(last_error_message) = strdup(buffer);I'm thinking about "fixing" this by leaking memory pointed by
PG(last_error_message) if php called when a timeout is pending
(timeouts are usually very rare, php processes will eventually be
restarted so this little memory waste won't have time to make any
impact.Is this issue fixed in modern php ? If so I would be grateful for some
information about the way it was done. This would save me a lot of
time trying to trigger
a non existing confition.I will try to reproduce this in a modern version of php.
It probably isn't. PG(last_error_message) is only modified in main.c, I
would try to use a separate arena for PG(last_error_message) and
PG(last_error_file). For instance it could be a static buffer reused during
the whole execution and extended with mmap(2) in the unlikely case it turns
out to be too small. I suspect it would also make the error handler faster,
as it would avoid the free() + malloc()
thank You I didn't notice that it is used only there, i will try to
use a static buffer.
I managed to produce a segfault on current php version (php heap
corruption), bug report https://bugs.php.net/bug.php?id=65674
spprintf() allocating memory is also not safe. I will try to fix this
by using a static buffer.
Thanks,
Michal Grzedzicki