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SubjectRe: [PATCH v2 0/5] [RFC] copy_strtok_from_user

On Thu, 26 Feb 2009, Ingo Molnar wrote:

> * Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > On Thu, 2009-02-26 at 00:32 -0500, Steven Rostedt wrote:
> > > > This is the second series of the uaccess code.
> > > >
> > > > Changes in v2:
> > > >
> > > > - moved probe_kernel_* functions to lib/uaccess.c
> > > >
> > > > - renamed copy_word_from_user to copy_strtok_from_user.
> > > >
> > > > - changed copy_strtok_from_user to pass in a delimiter string.
> > > > ftrace defines SPACE to be ' \t\r\n'.
> > > >
> > > > Ingo,
> > > > I added your copy right to lib/uaccess.c since git blame shows you
> > > > as the author of the probe_kernel_* code. Also, is it OK that I
> > > > added the "GPL v2" line in that file as well?
> > > >
> > > > Andrew,
> > > > Since you are, in essence, the memory maintainer, could you give
> > > > your Acked-by: to the copy_strtok_from_user code.
> > > >
> > > > The probe_kernel code is still EXPORT_SYMBOL_GPL, and I added
> > > > that too to copy_strtok_from_user. Are there any objections to that?
> > >
> > > I have to ask,..
> > >
> > > cant this be done with a regular copy_from_user() followed
> > > by a regular strtok()? Do we really have to combine all
> > > that?
> >
> > yes. Note that strsep() is the preferred API. (in fact it's
> > the only such string API that is in the kernel)
>
> btw., for larger strings we cannot really copy into the kernel i
> suspect. We'd have to kmalloc() and that adds overhead, etc.
>
> Nevertheless, copy_strsep_from_user() would be the symmetric API
> here.

not really. strsep stops at the first delimiter. strtok skips multiple
delimiters. I could implement the copy_strtok_from_user with a
copy_from_user (all of read), and then use strtok_r (reentrant version) to
find the next token. This would require implementing strtok_r for the
kernel.

Having the copy_strtok_from_user will handle the copying and the caller
would not need to worry about having a big enough buffer to hold
delimiters and tokens. The copy_strtok_from_user would copy first to an
internal buffer, then scan for tokens to copy into the callers buffer.
Then repeat until read or copy limits have been hit.

-- Steve



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