[FFmpeg-devel] [PATCH 1/2] avutil/random_seed: add av_random()

[James Almer]

On 7/4/2023 4:34 PM, Marton Balint wrote:
> 
> 
> On Tue, 4 Jul 2023, James Almer wrote:
> 
>> Uses the existing code for av_get_random_seed() to return a buffer with
>> cryptographically secure random data, or an error if none could be 
>> generated.
>>
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>> TODO: APIChanges entry and minor version bump.
>>
>> Also, if a new random.h header is prefered, i can move the prototype 
>> there.
>>
>> libavutil/random_seed.c | 46 ++++++++++++++++++++++++++---------------
>> libavutil/random_seed.h | 12 +++++++++++
>> 2 files changed, 41 insertions(+), 17 deletions(-)
>>
>> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
>> index 66dd504ef0..39fb27c5ad 100644
>> --- a/libavutil/random_seed.c
>> +++ b/libavutil/random_seed.c
>> @@ -46,20 +46,20 @@
>> #define TEST 0
>> #endif
>>
>> -static int read_random(uint32_t *dst, const char *file)
>> -{
>> #if HAVE_UNISTD_H
>> +static ssize_t read_random(uint8_t *dst, size_t len, const char *file)
>> +{
>>     int fd = avpriv_open(file, O_RDONLY);
>> -    int err = -1;
>> +    ssize_t err = -1;
>>
>> +    if (len > SSIZE_MAX)
>> +        return -1;
>>     if (fd == -1)
>>         return -1;
>> -    err = read(fd, dst, sizeof(*dst));
>> +    err = read(fd, dst, len);
>>     close(fd);
>>
>>     return err;
>> -#else
>> -    return -1;
>> #endif
>> }
>>
>> @@ -118,29 +118,41 @@ static uint32_t get_generic_seed(void)
>>     return AV_RB32(digest) + AV_RB32(digest + 16);
>> }
>>
>> -uint32_t av_get_random_seed(void)
>> +int av_random(uint8_t* buf, size_t len)
>> {
>> -    uint32_t seed;
>> -
>> #if HAVE_BCRYPT
>>     BCRYPT_ALG_HANDLE algo_handle;
>>     NTSTATUS ret = BCryptOpenAlgorithmProvider(&algo_handle, 
>> BCRYPT_RNG_ALGORITHM,
>>                                                MS_PRIMITIVE_PROVIDER, 0);
>>     if (BCRYPT_SUCCESS(ret)) {
>> -        NTSTATUS ret = BCryptGenRandom(algo_handle, (UCHAR*)&seed, 
>> sizeof(seed), 0);
>> +        NTSTATUS ret = BCryptGenRandom(algo_handle, (PUCHAR)buf, len, 
>> 0);
>>         BCryptCloseAlgorithmProvider(algo_handle, 0);
>>         if (BCRYPT_SUCCESS(ret))
>> -            return seed;
>> +            return 0;
>>     }
>> #endif
>>
>> #if HAVE_ARC4RANDOM
>> -    return arc4random();
>> +    arc4random_buf(buf, len);
>> +    return 0;
>> +#endif
>> +
>> +#if HAVE_UNISTD_H
>> +    if (read_random(buf, len, "/dev/urandom") == len)
>> +        return 0;
>> +    if (read_random(buf, len, "/dev/random")  == len)
>> +        return 0;
>> #endif
>>
>> -    if (read_random(&seed, "/dev/urandom") == sizeof(seed))
>> -        return seed;
>> -    if (read_random(&seed, "/dev/random")  == sizeof(seed))
>> -        return seed;
>> -    return get_generic_seed();
>> +    return AVERROR_INVALIDDATA;
> 
> Probably AVERROR(ENOSYS) is a more suiting error code for this.

Not if any of the functions above were called but failed to fill the buffer.

I could add return AVERROR(ENOSYS) to the HAVE_UNISTD_H block, and 
return AVERROR_INVALIDDATA outside.