Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v2] random: allow partial reads if later user copies fail | Date | Fri, 8 Apr 2022 08:11:35 +0000 |
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From: Jason A. Donenfeld > Sent: 08 April 2022 00:36 > > Rather than failing entirely if a copy_to_user() fails at some point, > instead we should return a partial read for the amount that succeeded > prior, unless none succeeded at all, in which case we return -EFAULT as > before.
I think you now return -EFAULT for a zero length read.
David
> > This makes it consistent with other reader interfaces. For example, the > following snippet for /dev/zero outputs "4" followed by "1": > > int fd; > void *x = mmap(NULL, 4096, PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); > assert(x != MAP_FAILED); > fd = open("/dev/zero", O_RDONLY); > assert(fd >= 0); > printf("%zd\n", read(fd, x, 4)); > printf("%zd\n", read(fd, x + 4095, 4)); > close(fd); > > This brings that same standard behavior to the various RNG reader > interfaces. > > While we're at it, we can streamline the loop logic a little bit. > > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Jann Horn <jannh@google.com> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > Changes v1->v2: > - Do partial copies within individual blocks, not just per-block, also > following how /dev/zero and ordinary filesystem files work. > > drivers/char/random.c | 22 ++++++++++++---------- > 1 file changed, 12 insertions(+), 10 deletions(-) > > diff --git a/drivers/char/random.c b/drivers/char/random.c > index e15063d61460..df43c5060f00 100644 > --- a/drivers/char/random.c > +++ b/drivers/char/random.c > @@ -523,8 +523,7 @@ EXPORT_SYMBOL(get_random_bytes); > > static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) > { > - ssize_t ret = 0; > - size_t len; > + size_t len, left, ret = 0; > u32 chacha_state[CHACHA_STATE_WORDS]; > u8 output[CHACHA_BLOCK_SIZE]; > > @@ -543,37 +542,40 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) > * the user directly. > */ > if (nbytes <= CHACHA_KEY_SIZE) { > - ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes; > + ret = nbytes - copy_to_user(buf, &chacha_state[4], nbytes); > goto out_zero_chacha; > } > > - do { > + for (;;) { > chacha20_block(chacha_state, output); > if (unlikely(chacha_state[12] == 0)) > ++chacha_state[13]; > > len = min_t(size_t, nbytes, CHACHA_BLOCK_SIZE); > - if (copy_to_user(buf, output, len)) { > - ret = -EFAULT; > + left = copy_to_user(buf, output, len); > + if (left) { > + ret += len - left; > break; > } > > - nbytes -= len; > buf += len; > ret += len; > + nbytes -= len; > + if (!nbytes) > + break; > > BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0); > - if (!(ret % PAGE_SIZE) && nbytes) { > + if (ret % PAGE_SIZE == 0) { > if (signal_pending(current)) > break; > cond_resched(); > } > - } while (nbytes); > + } > > memzero_explicit(output, sizeof(output)); > out_zero_chacha: > memzero_explicit(chacha_state, sizeof(chacha_state)); > - return ret; > + return ret ? ret : -EFAULT; > } > > /* > -- > 2.35.1
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