On 22-11-07 19:35, Ramagudi Naziir wrote:
taking a look at the system_call handler (entry_32.S), I saw that after every push to the stack the directive CFI_ADJUST_CFA_OFFSET 4 is used, and after every pop there is a CFI_ADJUST_CFA_OFFSET -4. why is that ?
These only exist for inserting some debugging information which normally a compiler would (could) take care of. CFI stands for Call Frame Information and helps a debugger create a reliable backtrace through functions.
This has potential value for looking at the kernel with debuggers such as GDB and NLKD and potential value inside the kernel itself when somebody ports a DWARF unwinder to the kernel proper (DWARF is the debugging format CFI is a part of) which google shows some are contemplating.
If you look at CFI_ADJUST_CFA_OFFSET you see it turn into an assembler pesudo op that you could, as it advices, learn more about in info as.
You also see that without CONFIG_UNWIND_INFO, they turn into whitespace and given that at least currently CONFIG_UNWIND_INFO is a dead config variable you may conclude that it's currently fairly safe not too worry about them. Robert Day tried to remove the macro defintions a while ago it seems so he may have further information in the form of people telling him why they wanted them to stay.
when do I have to put those directives after push and pop ?
When programking in pure assembly and after they've promoted themselves from no-op's to something substantial. Perhaps.
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