[User-mode-linux-devel] segfault around execve() and others when attaching gdb

[Yann D. <yd...@al...>]

Still using 2.4.0-test9 + um on a Debian "woody" host:

Linux bylbo 2.2.17 #1 Sun Sep 10 19:21:52 CEST 2000 i586 unknown

I compiled the um kernel with debugging support (.config attached) and
cannot attach to it without getting a segfault.


* debug from beggining:

Console output stops where I usually get init's startup message, which
is coherent with `um_execve ("/sbin/init")' in the traces.


$ ~/usr/src/umlinux[704]$ ./kernel-source-2.4.0-test9-um/linux debug ubd0=/home/dwitch/usr/src/umlinux/rootfs/root_fs_debian2.2_small umn=10.0.0.132 
tracing thread pid = 25507
Linux version 2.4.0-test9-1um (dwitch@bylbo) (gcc version 2.95.2 20000220 (Debian GNU/Linux)) #1 lun oct 16 22:20:01 CEST 2000
On node 0 totalpages: 4096
zone(0): 0 pages.
zone(1): 4096 pages.
zone(2): 0 pages.
Kernel command line: debug ubd0=/home/dwitch/usr/src/umlinux/rootfs/root_fs_debian2.2_small umn=10.0.0.132 root=/dev/ubd0
[...]
VFS: Mounted root (ext2 filesystem) readonly.
Mounted devfs on /dev


5400010
att 1
b start_kernel
c
GNU gdb 5.0
[Copyright...]

(gdb) att 1
Attaching to Pid 1
0x100b4d11 in ?? ()
(gdb) b start_kernel
No symbol table is loaded.  Use the "file" command.
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x10034a70 in ?? ()
(gdb) symbol-file kernel-source-2.4.0-test9-um/linux 
Reading symbols from kernel-source-2.4.0-test9-um/linux...done.
(gdb) bt
#0  0x10034a70 in padzero (elf_bss=1073818712)
    at /home/dwitch/usr/src/umlinux/kernel-source-2.4.0-test9-um/include/asm/arch/string.h:418
#1  0x10035349 in load_elf_interp (interp_elf_ex=0x50053d48, 
    interpreter=0x500e8180, interp_load_addr=0x50053d18) at binfmt_elf.c:320
#2  0x10035c2e in load_elf_binary (bprm=0x50053e1c, regs=0x0)
    at binfmt_elf.c:666
#3  0x10026262 in search_binary_handler (bprm=0x50053e1c, regs=0x0)
    at exec.c:809
#4  0x10026484 in do_execve (filename=0x100fbd2d "/sbin/init", 
    argv=0x1013a140, envp=0x1013a180, regs=0x0) at exec.c:902
#5  0x100aa48b in execve1 (file=0x100fbd2d "/sbin/init", argv=0x1013a140, 
    env=0x1013a180) at exec_kern.c:77
#6  0x100aa4cb in um_execve (file=0x100fbd2d "/sbin/init", argv=0x1013a140, 
    env=0x1013a180) at exec_kern.c:87
#7  0x100013b8 in init (unused=0x0) at init/main.c:768
#8  0x100afcf8 in new_thread_proc (t=0x50052000) at process_kern.c:128
(gdb) 


* attach to running kernel:

It takes a couple of seconds before the segfault comes, but it is
quite reproducible.

In this case, the console is still responsive - I can halt the system
with no apparent problem.

It appears the thread which gets the segfault is not the kernel itself

6c00010
GNU gdb 5.0
[...]

(gdb) att 1
Attaching to Pid 1
0x100bc111 in ?? ()
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x804b871 in ?? ()
(gdb) bt
#0  0x804b871 in ?? ()
#1  0x804cd86 in ?? ()
#2  0x804d0ba in ?? ()
#3  0x4002da42 in ?? ()
(gdb) symbol-file kernel-source-2.4.0-test9-um/linux
Reading symbols from kernel-source-2.4.0-test9-um/linux...done.
(gdb) bt
#0  0x804b871 in ?? ()
#1  0x804cd86 in ?? ()
#2  0x804d0ba in ?? ()
#3  0x4002da42 in ?? ()
(gdb) 


If I just have the login prompt when I attach, just hit 'c' after the
1st segfault which I cannot tell what thread/process got, then when
pressing RETURN at the login prompt (staying in getty IIRC) I get a
second segfault, in the kernel this time:

(gdb) bt
#0  0x10012154 in file_read_actor (desc=0x5019de2c, page=0x500371e0, offset=0, 
    size=25)
    at /home/dwitch/usr/src/umlinux/kernel-source-2.4.0-test9-um/include/asm/arch/string.h:202
#1  0x10011df9 in do_generic_file_read (filp=0x500e8720, ppos=0x500e8740, 
    desc=0x5019de2c, actor=0x100120f0 <file_read_actor>) at filemap.c:1009
#2  0x100121d7 in generic_file_read (filp=0x500e8720, 
    buf=0x40014000 <Address 0x40014000 out of bounds>, count=4096, 
    ppos=0x500e8740) at filemap.c:1140
#3  0x1001d0d7 in sys_read (fd=3, 
    buf=0x40014000 <Address 0x40014000 out of bounds>, count=4096)
    at read_write.c:133
#4  0x100acbc5 in execute_syscall (syscall=3, args=0x5019dee4)
    at syscall_kern.c:340
#5  0x100ad01b in syscall_handler (unused=0) at syscall_user.c:113
#6  0x100a9eb9 in fork_handler (sig=10) at process.c:96
#7  <signal handler called>
#8  0x100b4d11 in kill () at umn_kern.c:202


If then I attempt a real login, `execve("/bin/login")' fails in much
the same way as init did in the first example:

Program received signal SIGSEGV, Segmentation fault.
0x10034a70 in padzero (elf_bss=1073818712)
    at /home/dwitch/usr/src/umlinux/kernel-source-2.4.0-test9-um/include/asm/arch/string.h:418
418     __asm__ __volatile__(
(gdb) bt
#0  0x10034a70 in padzero (elf_bss=1073818712)
    at /home/dwitch/usr/src/umlinux/kernel-source-2.4.0-test9-um/include/asm/arch/string.h:418
#1  0x10035349 in load_elf_interp (interp_elf_ex=0x5019dc04, 
    interpreter=0x50d0be60, interp_load_addr=0x5019dbd4) at binfmt_elf.c:320
#2  0x10035c2e in load_elf_binary (bprm=0x5019dcd8, regs=0x0)
    at binfmt_elf.c:666
#3  0x10026262 in search_binary_handler (bprm=0x5019dcd8, regs=0x0)
    at exec.c:809
#4  0x10026484 in do_execve (filename=0x50ea9000 "/bin/login", 
    argv=0xbf7fed54, envp=0xbf7ffe94, regs=0x0) at exec.c:902
#5  0x100aa48b in execve1 (file=0x50ea9000 "/bin/login", argv=0xbf7fed54, 
    env=0xbf7ffe94) at exec_kern.c:77
#6  0x100aa519 in sys_execve (
    file=0x804a911 <Address 0x804a911 out of bounds>, argv=0xbf7fed54, 
    env=0xbf7ffe94) at exec_kern.c:101
#7  0x100acbc5 in execute_syscall (syscall=11, args=0x5019dee4)
    at syscall_kern.c:340
#8  0x100ad01b in syscall_handler (unused=0) at syscall_user.c:113
#9  0x100a9eb9 in fork_handler (sig=10) at process.c:96
#10 <signal handler called>
#11 0x100b4d11 in kill () at umn_kern.c:202
(gdb) 


-- 
Yann Dirson    <yd...@al...> |    Why make M$-Bill richer & richer ?
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