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sftpserver: Fix client messages being ignored if sent at a high rate

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When using OpenSSH scp to read files larger than a few hundred
kilobytes, downloads stall and never finish. A workaround is to
pass -Xnrequests=1 to scp, which will cause scp to only do a
single concurrent SFTP read request at a time.

The cause for the problem is that if SFTP client messages are
received at a high rate, sftp_channel_default_data_callback() will
potentially be called with multiple messages in the incoming data
buffer, but only the first message will be extracted and handled.

So add a loop to extract as many SFTP client messages as available
from the incoming data buffer.

Signed-off-by: Colin Baumgarten <colin.baumgarten@hubersuhner.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
This commit is contained in:
Colin Baumgarten
2026-03-11 16:08:54 +01:00
committed by Jakub Jelen
parent 9478de8082
commit 3154a4ab8d
1 changed files with 18 additions and 10 deletions
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28
src/sftpserver.c
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@@ -2104,7 +2104,7 @@ sftp_channel_default_data_callback(UNUSED_PARAM(ssh_session session),
sftp_session *sftpp = (sftp_session *)userdata;
sftp_session sftp = NULL;
sftp_client_message msg;
int decode_len;
uint32_t undecoded_len = len;
int rc;
if (sftpp == NULL) {
@@ -2113,17 +2113,25 @@ sftp_channel_default_data_callback(UNUSED_PARAM(ssh_session session),
}
sftp = *sftpp;
decode_len = sftp_decode_channel_data_to_packet(sftp, data, len);
if (decode_len == SSH_ERROR)
return SSH_ERROR;
do {
int decode_len =
sftp_decode_channel_data_to_packet(sftp, data, undecoded_len);
if (decode_len == SSH_ERROR) {
return SSH_ERROR;
}
msg = sftp_get_client_message_from_packet(sftp);
rc = process_client_message(msg);
sftp_client_message_free(msg);
if (rc != SSH_OK)
SSH_LOG(SSH_LOG_PROTOCOL, "process sftp failed!");
msg = sftp_get_client_message_from_packet(sftp);
rc = process_client_message(msg);
sftp_client_message_free(msg);
if (rc != SSH_OK) {
SSH_LOG(SSH_LOG_PROTOCOL, "process sftp failed!");
}
return decode_len;
undecoded_len -= decode_len;
data = (uint8_t *)data + decode_len;
} while (undecoded_len > 0);
return len;
}
#else
/* Not available on Windows for now */