Re: [Valgrind-developers] Does memcheck instrument the load/store in _start function?

[Niall D. <ndo...@bl...>]

> Under Linux, the first instruction executed in executable is in _start
function
> rather than in main function. I am wondering whether Valgrind instrument
the
> load/store in  any other functions except main function and its subroutine
(ex.
> _start function). According to Valgrind paper, all memory is marked as
> unaddressable except malloced. If Valgrind instrument memory check with
> load/store in _start function. It may report a false-positive error. How
does
> Valgrind deal with this problem?

I think you may misunderstand what valgrind is.

Valgrind emulates a CPU. It therefore can, and does, track all reads and
writes of memory across the process irrespective of what the code is.
Valgrind has no concept of main being different to _start, or indeed of
main, it just calls what ELF tells it to. If there are any false positives,
it's either a bug in valgrind (very rare) or your code is doing something
very unusual and probably should be doing it differently.

The only legitimate source of false positives in memcheck I'm aware of is
the magic cookie technique where one speculatively reads unknown memory
locations for the presence of a magic cookie. In this situation you should
not suppress the error, but you should instrument the code using valgrind's
macro support to tell valgrind what you're really doing.

Niall