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Subject: Re: [libssh2] libssh2_sftp_read* bug

Re: [libssh2] libssh2_sftp_read* bug

From: Daniel Stenberg <daniel_at_haxx.se>
Date: Tue, 13 Feb 2007 21:56:19 +0100 (CET)

On Mon, 12 Feb 2007, Lucas Newman wrote:

> I think I understand what is happening now. If a large read blocks in the
> middle, the chunk of data obtained is processed in transport.c to determine
> if a full packet was snagged. While processing half of a packet, if the
> data is not a multiple of the crypto block size, the extra bytes are
> discarded from the end of the chunk, and the remainder of the packet is
> missing those bytes, hence the MAC failure.
>
> To see this happening, add the following line to transport.c:441:
> fprintf(stderr, "Bytes being discarded: %d\n", numdecrypt % session-
> >remote.crypt->blocksize);
>
> If you are able to read a whole packet at once, you will never discard any
> bytes when decrypting, so that is why the behavior only appears when doing
> large, blocking reads.
>
> A solution would be to retain the extra bytes and just process them in the
> next iteration.

I must be stupid, but I read the code around line 441 and I can't see how the
bytes are being discarded.

Do you have a suggested patch to fix the problem you see?

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Received on 2007-02-13

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