Linux-kernel mailing list archive 1998-42,: Re: -EFAULT on invalid pointer

# Re: -EFAULT on invalid pointer

MOLNAR Ingo (mingo@chiara.csoma.elte.hu)
Tue, 20 Oct 1998 06:17:26 +0200 (CEST)

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On Tue, 20 Oct 1998, Chris Wedgwood wrote:

> buf = malloc(8192+4095) & -4095; /* two pages */
>
> mprotect(&buf[4096],4096,PROT_NONE); /* second page no good */
>
> read(fd,buf,8192);
>
>
> under 2.0.x, this will ALWAYS fail.
>
> under 2.1.x - this will never fail, it reads either the entire file
> if its less than 4k or the first 4k - advancing the file pointer.

this makes no difference to the memory management application i outlined.
2.0 and 2.1 simply has a different granularity of detecting memory faults,
but we have no information anyway where the fault occured, user-space has
to find out if it wants to deal with this. But returning no -EFAULT on a
fault makes it impossible (well, hard) for user-space to detect this.
User-space also has to deal with side-effects of failed syscalls. Tough.

> > ? we have not changed any semantics.
>
> see above. (the details might not be exact, I have code to show this
> though, I'll dig it up if you want)

(ok, i though you refered to the -EFAULT behavior on completely bad
pointers)

-- mingo

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