basic questions to strace

Lentes, Bernd bernd.lentes at
Fri Oct 7 17:06:11 UTC 2016

> On Fri, Oct 7, 2016 at 4:08 PM, Lentes, Bernd
> <bernd.lentes at> wrote:

>> Is that a segmentation fault ?
> Well, I assume so by "general protection" message from your excerpt,
> which is printed when there is unhandled SIGSEGV, iirc.

It is one. When i do a "strace -f /usr/sbin/nsrexecd 2>&1|less" these are the last lines:

[pid 14272] read(10, "4\277\267\233:Q\215\313\207\1e]\373\317\16\2755\375\251\371t\335\233q\223\214\350\tgN3\323"..., 48) = 48
[pid 14272] write(10, "\25\3\1\0 \34\247\3343z\\\377\264\313\273\323\363%\1\202\26\240v\236\177Hp\2724b\344s"..., 37) = 37
[pid 14272] read(10, "\25\3\1\0 ", 5)   = 5
[pid 14272] read(10, "\277M)\5\315\213&\7\371\22\306\335\305\246o\255\213A\352@\26H\312\331<lw[s\263\267\217", 32) = 32
[pid 14272] close(10)                   = 0
[pid 14272] --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=0} ---
[pid 14272] +++ killed by SIGSEGV (core dumped) +++
[pid 14271] +++ killed by SIGSEGV (core dumped) +++
[pid 14270] +++ killed by SIGSEGV (core dumped) +++
[pid 14269] +++ killed by SIGSEGV (core dumped) +++
+++ killed by SIGSEGV (core dumped) +++

>> Ah. So clone is called with all arguments but strace shows just the relevant ?
>> Does strace have this behaviour also with other system calls ?
> Yes. futex, fcntl, prctl, quotact, keyctl, ptrace are examples of such syscalls.

>> Sorry, i didn't read completely. Because the return code was != 0, i thought
>> it's an error code.
> Some syscalls return positive values in case of success, mostly the
> ones which provide fd as a result, but there are other examples of
> such behaviour. Only negative value indicates an error (in all cases
> i'm aware of, at least).

>> I installed the debian-kerbel via apt.
>> But it didn't install it completely, just downloaded a tar which i need to
>> extract manually.
>> Why do i have a package management ?
> There is linux-source package available in Debian. You can also do
> apt-get source linux-image and obtain deb-src which would also contain
> kernel sources.

I installed linux-source via apt but still had to decompress the kernel tar manually.

I have also core dumps which i don't know how to interpret. I can read the first few lines, but the rest
seems to be base64 (?) coded and i can't read it. Is there a way to make it human readable ?
I can offer it as a download (~400KB).

Thanks for your help, i appreciate that.
For me to keep a sharp eye on what the system is doing itself is fascinating.



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