Re: Possible bug in getdents64()?

[Alejandro Colomar]

On 2020-10-29 12:09, Florian Weimer wrote:
> * Alejandro Colomar via Libc-alpha:
>
> > [[ CC += linux-man, linux-kernel, libc-alpha, mtk ]]
> >
> > On 2020-10-28 20:26, Alejandro Colomar wrote:
> > > The manual page for getdents64() says the prototype should be the
> > > following:
> > >          int getdents64(unsigned int fd, struct linux_dirent64 *dirp,
> > >                       unsigned int count);
> > >
> > > Note the type of 'count': 'unsigned int'
> > > (usually a 32-bit unsigned integer).
> > > And the Linux kernel seems to use those types (fs/readdir.c:351):
> > > SYSCALL_DEFINE3(getdents64, unsigned int, fd,
> > >           struct linux_dirent64 __user *, dirent,
> > >           unsigned int, count)
> > > {
> > > ...
> > > }
> > > But glibc uses 'size_t' (usually a 64-bit unsigned integer)
> > > for the parameter 'count' (sysdeps/unix/linux/getdents64.c:25):
> > >
> > > /* The kernel struct linux_dirent64 matches the 'struct dirent64' type.  */
> > > ssize_t
> > > __getdents64 (int fd, void *buf, size_t nbytes)
> > > {
> > >     /* The system call takes an unsigned int argument, and some length
> > >        checks in the kernel use an int type.  */
> > >     if (nbytes > INT_MAX)
> > >       nbytes = INT_MAX;
> > >     return INLINE_SYSCALL_CALL (getdents64, fd, buf, nbytes);
> > > }
> > > libc_hidden_def (__getdents64)
> > > weak_alias (__getdents64, getdents64)
> > >
> > > Isn't it undefined behavior to pass a variable of a different
> > > (larger) type to a variadic function than what it expects?
> > > Is that behavior defined in this implementation?
> > > Wouldn't a cast to 'unsigned int' be needed?
>
> There is no variadic function involved here.  INLINE_SYSCALL_CALL takes
> care of the zero extension to the register internally, irrespective of
> the argument type.  (The register is of type long int or long long int,
> depending on the architecture.)

Hi Florian,

Thank you very much!

Alex

> Thanks,
> Florian