linux-kernel - Re: Possible bug in getdents64()?

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Message-ID: <01065580-8602-52e6-0cca-22d1aa20a540@gmail.com>
Date:   Thu, 29 Oct 2020 12:04:13 +0100
From:   Alejandro Colomar <colomar.6.4.3@...il.com>
To:     libc-help@...rceware.org
Cc:     linux-man <linux-man@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: Possible bug in getdents64()?

[[ 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?
> 
> 
> Thanks,
> 
> Alex