Two questions:
1. If I pass size 0 to kmalloc, what does it return?
2. What happens if I pass a null pointer as the destination parameter
to copy_from_user? Does copy_from_user handle it safely or will the
kernel seg fault?
Thanks for your answers,
Ken Ashcraft
Hi,
On Thu, 1 Nov 2001, Ken Ashcraft wrote:
> 1. If I pass size 0 to kmalloc, what does it return?
AFAIK size is always rounded up, so you get the smallest possible
allocation unit.
> 2. What happens if I pass a null pointer as the destination parameter
> to copy_from_user? Does copy_from_user handle it safely or will the
> kernel seg fault?
The kernel won't crash, but it might fail (depending on whether 0 is a
valid user space address or not).
bye, Roman
On Fri, 2 Nov 2001, Roman Zippel wrote:
> Hi,
>
> On Thu, 1 Nov 2001, Ken Ashcraft wrote:
>
> > 1. If I pass size 0 to kmalloc, what does it return?
>
> AFAIK size is always rounded up, so you get the smallest possible
> allocation unit.
>
> > 2. What happens if I pass a null pointer as the destination parameter
> > to copy_from_user? Does copy_from_user handle it safely or will the
> > kernel seg fault?
>
> The kernel won't crash, but it might fail (depending on whether 0 is a
> valid user space address or not).
>
> bye, Roman
>
>
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>
> > 2. What happens if I pass a null pointer as the destination parameter
> > to copy_from_user? Does copy_from_user handle it safely or will the
> > kernel seg fault?
>
> The kernel won't crash, but it might fail (depending on whether 0 is a
> valid user space address or not).
Why does it matter if 0 is a valid user space or not? If I make the call
copy_from_user(0, user_ptr, 4);
the null pointer is the kernel address, not the user address. Can you
clarify please?
Thanks
Ken
Hi,
Ken Ashcraft wrote:
> Why does it matter if 0 is a valid user space or not? If I make the call
>
> copy_from_user(0, user_ptr, 4);
>
> the null pointer is the kernel address, not the user address. Can you
> clarify please?
Sorry, I misunderstood you. The kernel address has to be a valid address
of course, otherwise the behavior is undefined.
bye, Roman
Ken Ashcraft wrote:
>
> > > 2. What happens if I pass a null pointer as the destination parameter
> > > to copy_from_user? Does copy_from_user handle it safely or will the
> > > kernel seg fault?
> >
> > The kernel won't crash, but it might fail (depending on whether 0 is a
> > valid user space address or not).
>
> Why does it matter if 0 is a valid user space or not? If I make the call
>
> copy_from_user(0, user_ptr, 4);
>
> the null pointer is the kernel address, not the user address. Can you
> clarify please?
copy_from_user uses the string move instruction on the x86, so the
exception code would assume the source faulted not the dest. It would
return -EFAULT instead of causing an oops.
--
Brian Gerst
Brian Gerst wrote:
>
> Ken Ashcraft wrote:
> >
> > > > 2. What happens if I pass a null pointer as the destination parameter
> > > > to copy_from_user? Does copy_from_user handle it safely or will the
> > > > kernel seg fault?
> > >
> > > The kernel won't crash, but it might fail (depending on whether 0 is a
> > > valid user space address or not).
> >
> > Why does it matter if 0 is a valid user space or not? If I make the call
> >
> > copy_from_user(0, user_ptr, 4);
> >
> > the null pointer is the kernel address, not the user address. Can you
> > clarify please?
>
> copy_from_user uses the string move instruction on the x86, so the
> exception code would assume the source faulted not the dest. It would
> return -EFAULT instead of causing an oops.
Err, would return non-zero instead of -EFAULT.
--
Brian Gerst