4:15 p.m.
Michal Karm Babacek created MODCLUSTER-583:
----------------------------------------------
Summary: mod_cluster segfaults in process_info() due to wrongly generated
assembler instruction movslq
Key: MODCLUSTER-583
URL: https://issues.jboss.org/browse/MODCLUSTER-583
Project: mod_cluster
Issue Type: Bug
Components: Native (httpd modules)
Affects Versions: 2.0.0.Alpha1, 1.2.13.Final, 1.3.6.Final
Environment: RHEL, gcc
Reporter: Michal Karm Babacek
Assignee: Michal Karm Babacek
Priority: Blocker
Affected code:
[1.3.x|https://github.com/modcluster/mod_cluster/blob/1.3.x/native/mod_man...]
[1.2.x|https://github.com/modcluster/mod_cluster/blob/1.2.x/native/mod_man...]
[master|https://github.com/modcluster/mod_proxy_cluster/blob/master/native...]
Martin Osvald from Red Hat:
{quote}
h3. Description of problem:
mod_cluster segfaults in process_info() due to wrongly generated assembler instruction
movslq
How reproducible:
intermittent
Steps to Reproduce:
TBD or N/A
h3. Actual results:
{code}
Core was generated by `/usr/sbin/httpd.worker'.
Program terminated with signal 11, Segmentation fault.
#0 process_info (r=0x7fc2840386a8, errtype=<value optimized out>) at
mod_manager.c:1150
1150 ap_rprintf(r, ",Elected: %d,Read: %d,Transfered: %d,Connected:
%d,Load: %d\n",
(gdb) bt
#0 process_info (r=0x7fc2840386a8, errtype=<value optimized out>) at
mod_manager.c:1150
#1 0x00007fc2b9d8344a in manager_handler (r=0x7fc2840386a8) at mod_manager.c:2705
#2 0x00007fc2bf54f640 in ap_run_handler (r=0x7fc2840386a8) at
/usr/src/debug/httpd-2.2.15/server/config.c:158
#3 0x00007fc2bf552efe in ap_invoke_handler (r=0x7fc2840386a8) at
/usr/src/debug/httpd-2.2.15/server/config.c:376
#4 0x00007fc2bf55e300 in ap_process_request (r=0x7fc2840386a8) at
/usr/src/debug/httpd-2.2.15/modules/http/http_request.c:282
#5 0x00007fc2bf55b178 in ap_process_http_connection (c=0x7fc2ac051d00) at
/usr/src/debug/httpd-2.2.15/modules/http/http_core.c:190
#6 0x00007fc2bf557148 in ap_run_process_connection (c=0x7fc2ac051d00) at
/usr/src/debug/httpd-2.2.15/server/connection.c:43
#7 0x00007fc2bf564052 in process_socket (thd=0x7fc2bffe8d30, dummy=<value optimized
out>) at /usr/src/debug/httpd-2.2.15/server/mpm/worker/worker.c:544
#8 worker_thread (thd=0x7fc2bffe8d30, dummy=<value optimized out>) at
/usr/src/debug/httpd-2.2.15/server/mpm/worker/worker.c:894
#9 0x00007fc2bddf99d1 in start_thread (arg=0x7fc2a51f4700) at pthread_create.c:301
#10 0x00007fc2bdb46a8d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115
(gdb)
{code}
h3. Expected results:
no segfault
h3.Additional info:
It looks like the produced assembler in function process_info() is wrong, there is a
movslq instruction leading to results not expected on C source code level...
{code}
# gdb /usr/sbin/httpd.worker
./dump_new/dump/core.httpd.worker.pid8422.usr48.sig11.tim1493787964
...
(gdb) f 0
#0 process_info (r=0x7fc2840386a8, errtype=<value optimized out>) at
mod_manager.c:1150
1150 ap_rprintf(r, ",Elected: %d,Read: %d,Transfered: %d,Connected:
%d,Load: %d\n",
(gdb) list
1145 #if AP_MODULE_MAGIC_AT_LEAST(20101223,1)
1146 proxystat = (proxy_worker_shared *) pptr;
1147 #else
1148 proxystat = (proxy_worker_stat *) pptr;
1149 #endif
1150 ap_rprintf(r, ",Elected: %d,Read: %d,Transfered: %d,Connected:
%d,Load: %d\n",
1151 (int) proxystat->elected, (int) proxystat->read, (int)
proxystat->transferred,
1152 (int) proxystat->busy, proxystat->lbfactor);
1153
1154 }
(gdb) i registers
rax 0x7fc26b59df58 140473001434968 <<<--- there should be
0x7fc2bf507f98 instead
rbx 0x7fc24c056b5c 140472475806556
rcx 0x7fc2840229a8 140473415117224
rdx 0xffffffffac096100 -1408671488
rsi 0x7fc2b9d85c00 140474318347264
rdi 0x7fc2840386a8 140473415206568
rbp 0x17 0x17
rsp 0x7fc2a51f3b40 0x7fc2a51f3b40
r8 0x3036203a6c74542c 3473999597862671404
r9 0x6873756c462c6666 7526488510160463462
r10 0x3031203a74696177 3472592223112618359
r11 0x0 0
r12 0x7fc2a51f3b98 140473970670488
r13 0x7fc2840386a8 140473415206568
r14 0x17 23
r15 0x431bde82d7b634db 4835703278458516699
rip 0x7fc2b9d7d90d 0x7fc2b9d7d90d <process_info+477>
eflags 0x203 [ CF IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
(gdb) disassemble /m $rip
...
1143 pptr = (char *) ou;
0x00007fc2b9d7d8f4 <+452>: mov 0x58(%rsp),%rax
1144 pptr = pptr + ou->offset;
1145 #if AP_MODULE_MAGIC_AT_LEAST(20101223,1)
1146 proxystat = (proxy_worker_shared *) pptr;
1147 #else
1148 proxystat = (proxy_worker_stat *) pptr;
0x00007fc2b9d7d903 <+467>: movslq 0x138(%rax),%rdx <<<--- this
brings wrong result into rdx
0x00007fc2b9d7d90a <+474>: add %rdx,%rax <<<--- which
is then added to rax, rax should normally hold address of ou + ou->offset
1149 #endif
1150 ap_rprintf(r, ",Elected: %d,Read: %d,Transfered: %d,Connected:
%d,Load: %d\n",
0x00007fc2b9d7d8f9 <+457>: lea 0x8300(%rip),%rsi # 0x7fc2b9d85c00
<<<--- the string ",Elected: %d,Read: %d,Transfered: %d,Connected: %d,Load:
%d\n"
0x00007fc2b9d7d900 <+464>: mov %r13,%rdi
<<<--- r parameter
=> 0x00007fc2b9d7d90d <+477>: mov 0xc0(%rax),%r9 <<<---
ap_rprintf params starting here proxystat->elected, ...
0x00007fc2b9d7d914 <+484>: mov 0x20(%rax),%r8
0x00007fc2b9d7d918 <+488>: mov 0x28(%rax),%rcx
0x00007fc2b9d7d91c <+492>: mov 0x30(%rax),%rdx
0x00007fc2b9d7d920 <+496>: mov 0x18(%rax),%eax
0x00007fc2b9d7d923 <+499>: mov %eax,(%rsp)
0x00007fc2b9d7d926 <+502>: xor %eax,%eax
0x00007fc2b9d7d928 <+504>: callq 0x7fc2b9d7c0f0 <ap_rprintf@plt>
...
(gdb)
{code}
from gdb it looks like proxystat contains correct address, but it only seems so as very
likely due to debugging info knowing where to look:
{code}
(gdb) p proxystat
$2 = (proxy_worker_stat *) 0x7fc2bf507f98
(gdb) p *proxystat
$11 = {
status = 0,
error_time = 0,
retries = 0,
lbstatus = 0,
lbfactor = 0,
transferred = 0,
read = 0,
elected = 0,
route = '\000' <repeats 63 times>,
redirect = '\000' <repeats 63 times>,
context = 0x0,
busy = 206158430208,
lbset = 1818454389
}
(gdb)
{code}
but actually computed value for proxystat by assembler is wrongly 0x7fc26b59df58 instead
of 0x7fc2bf507f98:
{code}
(gdb) x/x $rax
0x7fc26b59df58: Cannot access memory at address 0x7fc26b59df58
(gdb)
{code}
address of ou and offset value:
{code}
(gdb) p ou
$5 = (nodeinfo_t *) 0x7fc2bf507e58
(gdb) p /x ou->offset
$7 = 0x140
(gdb)
{code}
their correct sum:
{code}
(gdb) p /x 0x7fc2bf507e58 + 0x140
$9 = 0x7fc2bf507f98
(gdb)
{code}
which is the same as already mentioned proxystat using debuginfo:
{code}
(gdb) p proxystat
$2 = (proxy_worker_stat *) 0x7fc2bf507f98
(gdb)
{code}
so the below 2 lines in process_info() lead to computing it wrongly and they will have to
be rewritten:
{code}
1097 static char * process_info(request_rec *r, int *errtype)
1098 {
...
1141 pptr = (char *) ou;
1142 pptr = pptr + ou->offset;
{code}
{quote}
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