Re: sftp_write failure question

[Jakub Jelen <jjelen@xxxxxxxxxx>]

Hi,
what version of libssh do you use?

The sftp reads and writes are capped to some size, which is reported
by limits@xxxxxxxxxxx SFTP extension and which might differ between
read and write

https://github.com/openssh/openssh-portable/commit/788cbc5b74a53956ba9fff11e1ca506271a3597f

The value 262120 is clearly over the 256*1024-1024, which is max write
size in OpenSSH sftp server reported by limits:

https://github.com/openssh/openssh-portable/blob/master/sftp-server.c#L1542

The latest version (master and soon to be released 0.11) should
already handle this and not attempt to write more than limits.

In any case, if you are writing some new application using SFTP, I can
recommend you to have a look into the new async SFTP, which should
provide much better throughput (available in master and
soon-to-be-released 0.11):

https://api.libssh.org/master/libssh_tutor_sftp_aio.html

Jakub

On Tue, Jul 16, 2024 at 2:31 AM <reid.thompson@xxxxxxxxxxxxxxx> wrote:
>
> with code of the format below ( pulled from the examples ) where source
> is say 75MB in size.
>
> ...snip...
>     while ((len = sftp_read(source, data, 4096)) > 0) {
>         if (sftp_write(to, data, len) != len) {
>             fprintf(stderr, "Error writing %d bytes: %s\n",
>                     len, ssh_get_error(session));
>             sftp_close(to);
>             sftp_close(fichier);
>             goto end;
>         }
>     }
> ...snip...
>
> read/write will successfully transfer the complete file.
>
> if the code is altered to
>
> ...snip...
> BUF_SIZE=xxxxx
> char data[BUF_SIZE] = {0};
> ...
>     while ((len = sftp_read(source, data, BUF_SIZE)) > 0) {
>         if (sftp_write(to, data, len) != len) {
>             fprintf(stderr, "Error writing %d bytes: %s\n",
>                     len, ssh_get_error(session));
>             sftp_close(to);
>             sftp_close(fichier);
>             goto end;
>         }
>     }
> ...snip...
>
> read/write will succeed until the value of BUF_SIZE is set to 262120 or
> greater.
> at which point sftp_write will write BUF_SIZE and sftp_packet_read will
> fail with "Received EOF while reading sftp packet size" and the SFTP
> server will log a bad message error:
>     "Jul 15 17:28:28 endpoint sftp-server[1537206]: error: bad message from 100.98.195.26 local user rthompso"
>
> Is this expected/is this an internal limit to the amount that can be
> written per invocation?
> Is there a way to increase the amount of data that can be successfully
> written per invocation?
>
>
> Thanks,
> Reid
>
>
>