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

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Date:   Thu, 29 Oct 2020 12:09:36 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Alejandro Colomar via Libc-alpha <libc-alpha@...rceware.org>
Cc:     libc-help@...rceware.org,
        Alejandro Colomar <colomar.6.4.3@...il.com>,
        linux-man <linux-man@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: Possible bug in getdents64()?

* 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.)

Thanks,
Florian
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