[PATCH] [libc++] Don't return uninitialized data from random_device::operator()

[Marshall Clow]

On May 27, 2014, at 11:44 AM, David Majnemer <david.majnemer at gmail.com> wrote:
> On Tue, May 27, 2014 at 7:45 AM, Marshall Clow <mclow.lists at gmail.com> wrote:
> 
> On May 20, 2014, at 2:25 AM, David Majnemer <david.majnemer at gmail.com> wrote:
> 
>> Oops, sent out the wrong version of this patch.  Attached is what I intended to send.
>> 
>> 
>> On Tue, May 20, 2014 at 12:28 AM, David Majnemer <david.majnemer at gmail.com> wrote:
>> random_device::operator() as currently implemented does not correctly handle errors returned by read.  This can result in it returning uninitialized data.
>> 
>> To fix this, wrap the call to read in a loop.
> 
> I like this; but can you think of any way to test it?
> 
> I've added a test for the EOF case.
> 
> I have written a test for EINTR case but I did not included it because it's inherently not reliable. It checks the output of operator() to see if signals resulted in us getting a lot of zero return results.

I get a couple of signedness warnings when building:

+ for FILE in '../src/*.cpp'
+ /Sources/LLVM/bin/bin/clang++ -c -g -Os -arch i386 -arch x86_64 -nostdinc++ -std=c++11 -fstrict-aliasing -Wall -Wextra -Wshadow -Wconversion -Wpadded -Wstrict-aliasing=2 -Wstrict-overflow=4 -I../include ../src/random.cpp
../src/random.cpp:78:14: warning: implicit conversion changes signedness: 'ssize_t' (aka 'long') to 'unsigned long' [-Wsign-conversion]
        i += s;
          ~~ ^
1 warning generated.
../src/random.cpp:78:14: warning: implicit conversion changes signedness: 'ssize_t' (aka 'long') to 'unsigned long' [-Wsign-conversion]
        i += s;
          ~~ ^
1 warning generated.

I get that read can only return -1, 0, and positive numbers, and you’re already checking for -1 and 0, so I think that you can safely cast s to a size_t before doing the addition.


Also, from a readability standpoint, wouldn’t a while loop be better than the for loop?
Something like this:

    while (n > 0)
    {
        ssize_t s = read(__f_, p, n);
        if (s == 0)
            __throw_system_error(ENODATA, "random_device got EOF");
        if (s == -1)
        {
            if (errno != EINTR)
                __throw_system_error(errno, "random_device got an unexpected error");
            continue;
        }
        i -= (size_t) s;
        p += (size_t) s;
    }


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