| > That way the location of the .text section becomes reachable to the running program via the section header table. Not sure why you would want to find .text in the program, but if you do, the linker (at least ld.bfd and ld.lld do) adds the symbols __executable_start and _etext, which surround the program code. Using linker-resolved symbols is much more reliable than parsing section headers. > In addition to that, they must also be sorted! For some weird reason For efficiency and simplicity when loading. Re your article: > Unless I can figure out a way to move the program header table to the end of the file without breaking everything This should be doable, but you need to make sure that the program headers are mapped to memory (i.e., completely covered by a PT_LOAD segment). I do wonder why you use program headers, though: depending on your exact use case, it might be easier to link against an object file containing only data (e.g., from objcopy) and use symbols instead. |
Not sure either, I just used the example I read in your post. Now I'm curious about why someone would want to do that. Maybe to make the section writable and patch the code at runtime?
> __executable_start and _etext, which surround the program code.
You're right! I did see those symbols when I dumped ld's default linker script. Completely forgot about them.
> This should be doable, but you need to make sure that the program headers are mapped to memory (i.e., completely covered by a PT_LOAD segment).
I'll keep this in mind when I try it again. At the time I got pretty frustrated because it was pretty hard to debug and figure out why it was failing. The mold solution was like a light at the end of the tunnel for me.
> I do wonder why you use program headers, though: depending on your exact use case, it might be easier to link against an object file containing only data (e.g., from objcopy) and use symbols instead.
Objcopy was ths first thing I tried! Even asked a question about it on stack overflow.
https://stackoverflow.com/q/77468641
Long story short, by default the sections aren't covered by a PT_LOAD segment and so they are unreachable. I wanted the program to work even if the symbols were not defined which is why I tried to find it in the table at runtime.