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Re: SIGILL under gdb for AIX 64 bit binaries
- From: Jonas Maebe <jonas dot maebe at elis dot ugent dot be>
- To: navin p <navinp1281 at gmail dot com>
- Cc: gdb at sourceware dot org
- Date: Mon, 15 Dec 2014 10:44:51 +0100
- Subject: Re: SIGILL under gdb for AIX 64 bit binaries
- Authentication-results: sourceware.org; auth=none
- References: <CAKVd3pUAfRxcpVRbxoP+eWKqv9azQHzYod2qdooNG4bwPCDxVg at mail dot gmail dot com>
On 15 Dec 2014, at 09:12, navin p wrote:
When i run a program on command line it runs fine. But when i
run it gdb it gives me a SIGILL.
I know that is a problem with the code because dbx for AIX also
gives the SIGILL instruction.
But i've been not able to convince people in my organization it
is a problem with code.
Executing an instruction is a standard technique to determine whether
the cpu supports it (e.g. to detect whether an ARM cpu supports
prefetching instructions). The way this works is that the program
installs a signal handler for the SIGILL signal, executes the
potentially unsupported instruction, and sets a global variable inside
the signal handler, and then returns to the instruction after the
unsupported instruction. So by checking that variable after attempting
to execute that instruction, you can know whether the cpu supported it
or not.
What they do is they 'c' (continue) in gdb when it hits a
SIGILL and keep on continuing 2 times until it crashes somewhere else
in the code.
I wanted to know these 2 questions :
1) Once it hits a SIGILL can we continue and go to the next
crash and so on ? Is this valid .
2) How does gdb allow to continue on a SIGILL ?
type
handle SIGILL
You will see something like
Signal Stop Print Pass to program Description
SIGILL Yes Yes Yes Illegal instruction
"Stop = Yes" means that gdb will stop when the program triggers a
SIGILL. "Print = Yes" means that gdb will print a message telling you
that it stopped because of a SIGILL. "Pass to program = Yes" means
that after you continue the execution, the SIGILL will be passed on to
the program itself, so that its SIGILL handler will be executed. If it
did not install a SIGILL handler, it will simply be killed.
This means that if your program keeps executing once you continue
after receiving a SIGILL, it handled the SIGILL in a signal handler
and continued normally. As a result, it's probably expected behaviour.
Jonas