Apparent signal handling errors recently introduced
Greetings! GCL and its compiled programs (maxima, acl2, axiom) can
make use of a more efficient garbage collection algorithm by
mprotecting certain pages read only, trapping segmentation faults, and
then reprotecting the pages read write as necessary. This has been
working on alpha for a long time. acl2 in particular was last build
on 20021210 on lully successfully using this mechanism. I've made
some minor modifications to the acl2 package, and now the build breaks
on alpha only, apparently.
Debugging on escher shows that apparently the stack is damaged in such
a way as the program cannot determine where to return after the signal
handler, in particular the return is one stack level up from where the
segmentation signal was caught. Is there any recent kernel change
that could have caused this?
Here is a gdb session:
Starting program: /home/camm/acl2-2.7/raw_nsaved_acl2 /home/camm/gcl-2.6.1/unixport/ <init_nsaved_acl2.lsp
GCL (GNU Common Lisp) April 1994 131072 pages
Building symbol table for /home/camm/acl2-2.7/raw_nsaved_acl2 ..
loading /home/camm/gcl-2.6.1/unixport/../lsp/gcl_export.lsp
...
[GC for 5242 RELOCATABLE-BLOCKS pages..(T=764).GC finished]
[SGC on]
Program received signal SIGSEGV, Segmentation fault.
0x0000000120667b8c in LI20 (V83=0x120d74570) at gcl_sloop.c:530
530 bds_bind(VV[34],(V83));
(gdb) bt 5
#0 0x0000000120667b8c in LI20 (V83=0x120d74570) at gcl_sloop.c:530
#1 0x0000000120666ad8 in L7 () at gcl_sloop.c:230
#2 0x00000001204eb984 in funcall (fun=0x120b51380) at eval.c:173
#3 0x00000001205336ac in IapplyVector (fun=0x120b51380, nargs=2,
base=0x12084ea10) at nfunlink.c:239
#4 0x00000001204ef2f8 in fLfuncall (fun=0x120b51380) at eval.c:1140
(More stack frames follow...)
(gdb) up
#1 0x0000000120666ad8 in L7 () at gcl_sloop.c:230
230 base[3]= (*(LnkLI247))(base[2]);
(gdb) p base
$35 = (object *) 0x12084ea10
(gdb) p &base
$36 = (object **) 0x11fffe6f8
(gdb) c
Continuing.
Breakpoint 1, memprotect_handler (sig=11, code=4831830752, scp=0x11fffe360,
addr=0x0) at sgbc.c:1404
1404 int j=page_multiple;
(gdb) p *(object **) 0x11fffe6f8
$37 = (union lispunion **) 0x12084ea10
(gdb) display *(object **) 0x11fffe6f8
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb) n
1408 faddr=GET_FAULT_ADDR(sig,code,scp,addr);
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1409 debug_fault = (long) faddr;
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1413 if (faddr >= core_end || (unsigned long)faddr < DBEGIN) {
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1422 p = page(faddr);
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1423 p = ROUND_DOWN_PAGE_NO(p);
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1424 if (p >= first_protectable_page
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1433 mprotect(pagetochar(p),page_multiple * PAGESIZE, PROT_READ_WRITE);
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1434 while (--j >= 0)
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb) p j
$38 = 1
(gdb) n
1435 sgc_type_map[p+j] = sgc_type_map[p+j] | SGC_TEMP_WRITABLE;
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1434 while (--j >= 0)
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb)
1457 }
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb) bt
#0 memprotect_handler (sig=11, code=4831830752, scp=0x11fffe360, addr=0x0)
at sgbc.c:1457
#1 0x000002000022c858 in __open_catalog () from /lib/libc.so.6.1
#2 0x0000000120666ad8 in L7 () at gcl_sloop.c:230
(gdb) c
Continuing.
Program received signal SIGSEGV, Segmentation fault.
0x0000000120666ae4 in L7 () at gcl_sloop.c:230
230 base[3]= (*(LnkLI247))(base[2]);
1: *(union lispunion ***) 4831831800 = (union lispunion **) 0x12084ea10
(gdb) bt
#0 0x0000000120666ae4 in L7 () at gcl_sloop.c:230
(gdb) p &base
$39 = (object **) 0x12080b74c
(gdb) p base[0]
Cannot access memory at address 0x20add6c800000000
(gdb)
Take care,
--
Camm Maguire camm@enhanced.com
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