Date: Mon, 23 May 2016 01:10:48 +0200 From: Joerg Sonnenberger <joerg@bec.de> To: Conrad Meyer <cem@FreeBSD.org> Cc: 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: <20160522231048.GA25503@britannica.bec.de> In-Reply-To: <CAG6CVpWb7nvX%2BLFpLizkSx8Y-deXfXiWi=rL56iGZ71YPhmLbw@mail.gmail.com> References: <b2515cae-b75d-66e9-4207-3cf100ab3ab0@erdgeist.org> <CAG6CVpWb7nvX%2BLFpLizkSx8Y-deXfXiWi=rL56iGZ71YPhmLbw@mail.gmail.com>
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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]. There are a lot of valid reasons for not doing arbitrarily large operations, especially since a certain amount of atomicity is expected for local IO. That doesn't mean that it can't be turned into a short read/write instead. Erroring out seeems completely unjustified. Joerg
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