Re: Syscall changes registers beyond %eax, on linux-i386

[Brian Gerst]

Petr Vandrovec wrote:
> On 19 Sep 02 at 13:22, Richard B. Johnson wrote:
> 
> 
>>>>>A short snippet of sys_poll, with irrelavant data removed.
>>>>>
>>>>>sys_poll(struct pollfd *ufds, .. , ..) {
>>>>>   ...
>>>>>   ufds++;
>>>>>   ...
>>>>
>>Well which one?  Here is an ioctl(). It certainly modifies one
>>of its parameter values.
> 
> 
> poll(), as was already noted. Program below should
> print same value for B= and F=, but it reports f + 8*c instead
> (where c = number of filedescriptors passed to poll).
> 
> And you must call it from assembly, as your calls to getpid() or
> ioctl() (or poll()) are wrapped in libc - and glibc's code begins with
> push %ebx because of %ebx is used by -fPIC code.
> 
> It is questinable whether we should try to not modify parameters
> passed into functions. It is definitely nice behavior, but I think
> that we should only guarantee that syscalls do not modify unused
> registers.
>                                                     Petr Vandrovec
>                                                     [e-mail]

Now that I've thought about it more, I think the best solution is to go 
through all the syscalls (a big job, I know), and declare the parameters 
as const, so that gcc knows it can't modify them, and will throw a 
warning if we try.

--
				Brian Gerst