Have you ever wanted to embed GDB breakpoints in C source code?
int main() {
printf("Hello,\n");
EMBED_BREAKPOINT;
printf("world!\n");
EMBED_BREAKPOINT;
return 0;
}
One way is to directly insert your CPU's breakpoint instruction. On x86:
#define EMBED_BREAKPOINT asm volatile ("int3;")
There are at least two problems with this approach:
They aren't real GDB breakpoints. You can't disable them, count how many times they've been hit, etc.
If you run the program outside GDB, the breakpoint instruction will crash your process.
Here is a small hack which solves both problems:
#define EMBED_BREAKPOINT \
asm("0:" \
".pushsection embed-breakpoints;" \
".quad 0b;" \
".popsection;")
We place a local label into the instruction stream, and then save its address in the embed-breakpoints linker section.
Then we need to convert these addresses into GDB breakpoint commands. I wrote a tool that does this, as a wrapper for the gdb command. Here's how it works, on our initial example:
$ gcc -g -o example example.c
$ ./gdb-with-breakpoints ./example
Reading symbols from example...done.
Breakpoint 1 at 0x4004f2: file example.c, line 8.
Breakpoint 2 at 0x4004fc: file example.c, line 10.
(gdb) run
Starting program: example
Hello,
Breakpoint 1, main () at example.c:8
8 printf("world!\n");
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y 0x00000000004004f2 in main at example.c:8
breakpoint already hit 1 time
2 breakpoint keep y 0x00000000004004fc in main at example.c:10
If we run the program normally, or in GDB without the wrapper, the EMBED_BREAKPOINT statements do nothing. The breakpoint addresses aren't even loaded into memory, because the embed-breakpoints section is not marked as allocatable.
You can find all of the code on GitHub under a BSD license. I've done only minimal testing, but I hope it will be a useful debugging tool for someone. Let me know if you find any bugs or improvements. You can comment here, or find my email address on GitHub.
I'm not sure about the decision to write the GDB wrapper in C using BFD. I also considered Haskell and elf, or Python and the new pyelftools. One can probably do something nicer using the GDB Python API, which was added a few years ago.
This code depends on a GNU toolchain: it uses GNU C extensions, GNU assembler syntax, and BFD. The GDB wrapper uses the Linux proc filesystem, so that it can pass to GDB a temporary file which has already been unlinked. You could port it to other UNIX systems by changing the tempfile handling. It should work on a variety of CPU architectures, but I've only tested it on 32- and 64-bit x86.
