mirror of
https://github.com/yuzu-emu/breakpad
synced 2024-11-22 20:13:51 +00:00
0ff15b41ed
BUG=none R=mark CC=google-breakpad-dev@googlegroups.com Review URL: https://codereview.chromium.org/1357773004 . Patch from Andy Bonventre <andybons@chromium.org>.
47 lines
2.2 KiB
Markdown
47 lines
2.2 KiB
Markdown
# Introduction
|
|
|
|
Linux implements its userland-to-kernel transition using a special library
|
|
called linux-gate.so that is mapped by the kernel into every process. For more
|
|
information, see
|
|
|
|
http://www.trilithium.com/johan/2005/08/linux-gate/
|
|
|
|
In a nutshell, the problem is that the system call gate function,
|
|
kernel\_vsyscall does not use EBP to point to the frame pointer.
|
|
|
|
However, the Breakpad processor supports special frames like this via STACK
|
|
lines in the symbol file. If you look in src/client/linux/data you will see
|
|
symbol files for linux-gate.so for both Intel & AMD(the implementation of
|
|
kernel\_vsyscall changes depending on the CPU manufacturer). When processing
|
|
minidumps from Linux 2.6, having these symbol files is necessary for walking the
|
|
stack for crashes that happen while a thread is in a system call.
|
|
|
|
If you're just interested in processing minidumps, those two symbol files should
|
|
be all you need!
|
|
|
|
# Details
|
|
|
|
The particular details of understanding the linux-gate.so symbol files can be
|
|
found by reading about STACK lines inside
|
|
src/common/windows/pdb\_source\_line\_writer.cc, and the above link. To
|
|
summarize briefly, we just have to inform the processor how to get to the
|
|
previous frame when the EIP is inside kernel\_vsyscall, and we do that by
|
|
telling the processor how many bytes kernel\_vsyscall has pushed onto the stack
|
|
in it's prologue. For example, one of the symbol files looks somewhat like the
|
|
following:
|
|
|
|
MODULE Linux x86 random\_debug\_id linux-gate.so PUBLIC 400 0 kernel\_vsyscall
|
|
STACK WIN 4 100 1 1 0 0 0 0 0 1
|
|
|
|
The PUBLIC line indicates that kernel\_vsyscall is at offset 400 (in bytes) from
|
|
the beginning of linux-gate.so. The STACK line indicates the size of the
|
|
function(100), how many bytes it pushes(1), and how many bytes it pops(1). The
|
|
last 1 indicates that EBP is pushed onto the stack before being used by the
|
|
function.
|
|
|
|
# Warnings
|
|
|
|
These functions might change significantly depending on kernel version. In my
|
|
opinion, the actual function stack information is unlikely to change frequently,
|
|
but the Linux kernel might change the address of kernel\_vsyscall w.r.t the
|
|
beginning of linux-gate.so, which would cause these symbol files to be invalid.
|