How do you read a segfault kernel log message

Reference: How do you read a segfault kernel log message

This can be a very simple question, I'm am attempting to debug an application which generates the following segfault error in the kern.log

kernel: myapp[15514]: segfault at 794ef0 ip 080513b sp 794ef0 error 6 in myapp[8048000+24000]

Here are my questions:

Is there any documentation as to what are the diff error numbers on segfault, in this instance it is error 6, but i've seen error 4, 5

What is the meaning of the information at bf794ef0 ip 0805130b sp bf794ef0 and myapp[8048000+24000]?

So far i was able to compile with symbols, and when i do a x 0x8048000+24000 it returns a symbol, is that the correct way of doing it? My assumptions thus far are the following:

sp = stack pointer?
ip = instruction pointer
at = ????
myapp[8048000+24000] = address of symbol?




Based on my limited knowledge, your assumptions are correct.

sp = stack pointer
ip = instruction pointer
myapp[8048000+24000] = address

If I were debugging the problem I would modify the code to produce a core dump or log a stack backtrace on the crash. You might also run the program under (or attach) GDB.

The error code is just the architectural error code for page faults and seems to be architecture specific. They are often documented in arch/*/mm/fault.c in the kernel source. My copy of Linux/arch/i386/mm/fault.c has the following definition for error_code:

bit 0 == 0 means no page found, 1 means protection fault
bit 1 == 0 means read, 1 means write
bit 2 == 0 means kernel, 1 means user-mode

My copy of Linux/arch/x86_64/mm/fault.c adds the following:

bit 3 == 1 means fault was an instruction fetch


Answer:

When the report points to a program, not a shared library

Run addr2line -e myapp 080513b (and repeat for the other instruction pointer values given) to see where the error is happening. Better, get a debug-instrumented build, and reproduce the problem under a debugger such as gdb.
If it's a shared library

In the libfoo.so[NNNNNN+YYYY] part, the NNNNNN is where the library was loaded. Subtract this from the instruction pointer (ip) and you'll get the offset into the .so of the offending instruction. Then you can use objdump -DCgl libfoo.so and search for the instruction at that offset. You should easily be able to figure out which function it is from the asm labels. If the .so doesn't have optimizations you can also try using addr2line -e libfoo.so .
What the error means

Here's the breakdown of the fields:

address - the location in memory the code is trying to access (it's likely that 10 and 11 are offsets from a pointer we expect to be set to a valid value but which is instead pointing to 0)
ip - instruction pointer, ie. where the code which is trying to do this lives
sp - stack pointer
error - Architecture-specific flags; see arch/*/mm/fault.c for your platform.