Date: Sun, 22 May 2016 16:23:32 -0700 From: Conrad Meyer <cem@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Conrad Meyer <cem@freebsd.org>, Dirk Engling <erdgeist@erdgeist.org>, FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: read(2) and thus bsdiff is limited to 2^31 bytes Message-ID: <CAG6CVpXu9Atw4CV1dh5HMu1w43pTJ0iYZF4He_qzm_Ni0VhgiQ@mail.gmail.com> In-Reply-To: <20160522230942.GP89104@kib.kiev.ua> References: <b2515cae-b75d-66e9-4207-3cf100ab3ab0@erdgeist.org> <CAG6CVpWb7nvX%2BLFpLizkSx8Y-deXfXiWi=rL56iGZ71YPhmLbw@mail.gmail.com> <20160522230942.GP89104@kib.kiev.ua>
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On Sun, May 22, 2016 at 4:09 PM, Konstantin Belousov <kostikbel@gmail.com> wrote: > On Sun, May 22, 2016 at 03:56:33PM -0700, Conrad Meyer wrote: >> On Sun, May 22, 2016 at 1:54 PM, Dirk Engling <erdgeist@erdgeist.org> wrote: >> > When trying to bsdiff two DVD images, I noticed it failing due to >> > read(2) returning EINVAL to the tool. man 2 read says, this would only >> > happen for a negative value for fildes, which clearly was not true. >> >> Actually, it's documented at the very bottom of the first section: >> >> ERRORS >> The read(), readv(), pread() and preadv() system calls will succeed >> unless: >> ... >> [EINVAL] The value nbytes is greater than INT_MAX. >> >> It does seem silly to me given nbytes is a size_t. I think it should >> error if nbytes is greater than SSIZE_T_MAX, but on platforms where >> size_t is larger than int (e.g. amd64) it shouldn't error for nbytes >> in [INT_MAX, SSIZE_T_MAX - 1]. > It does not look silly to me, due to the typical > if (read() < 0) > checks in the code. Even > if (read() == -1) > is vulnerable. read(2) returns ssize_t; SSIZE_MAX is not a negative result. I agree that nbytes in [SSIZE_MAX+1, SIZE_MAX] should be disallowed (negative ssize_t value after cast from size_t). > >> >> As far as I can tell, this INT_MAX behavior is not required by POSIX. > From POSIX page for read(): > RETURN VALUE > Upon successful completion, these functions shall return a non-negative integer indicating the > number of bytes actually read. Otherwise, the functions shall return -1 and set errno to indicate > the error. There is a difference between int and ssize_t. They have different ranges on e.g. amd64. Best, Conrad
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