[pve-devel] [RFC PATCH http-server] fix #6230: increase allowed post size

[Dominik Csapak <d.csapak@proxmox.com>]

In some situations, e.g. having a large resource mapping, the UI can
generate a request that is bigger than the current limit of 64KiB.

Our files in pmxcfs can grow up to 1 MiB, so theoretically, a single
mapping can grow to that size. In practice, a single entry will have
much less. In #6230, a user has a mapping with about ~130KiB.

Increase the limit to 512KiB so we have a bit of buffer left.

We have to also increase the 'rbuf_max' size here, otherwise the request
will fail (since the buffer is too small for the request).
Since the post limit and the rbuf_max are tightly coupled, let it
reflect that in the code. To do that sum the post size + max header
size there.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
---
sending as RFC because:
* not sure about the rbuf_max calculation, but we have to increase it
  when we increase $limit_max_post. (not sure how much is needed exactly)
* ther are alternative ways to deal with that, but some of those are vastly
  more work:
  - optimize the pci mapping to reduce the number of bytes we have to
    send (e.g. by reducing the property names, or somehow magically
    detect devices that belong together)
  - add a new api for the mappings that can update the entries without
    sending the whole mapping again (not sure if we can make this
    backwards compatible)
  - ignore the problem and simply tell the users to edit the file
    manually (I don't like this one...)

also, I tried to benchmark this, but did not find a tool that does this
in a good way (e.g. apachebench complained about ssl, and i couldn't get
it to work right). @Thomas you did such benchmarks laft according to git
log, do you remember what you used then?

 src/PVE/APIServer/AnyEvent.pm | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/PVE/APIServer/AnyEvent.pm b/src/PVE/APIServer/AnyEvent.pm
index 8a52836..43ced75 100644
--- a/src/PVE/APIServer/AnyEvent.pm
+++ b/src/PVE/APIServer/AnyEvent.pm
@@ -52,7 +52,7 @@ use PVE::APIServer::Utils;
 
 my $limit_max_headers = 64;
 my $limit_max_header_size = 8*1024;
-my $limit_max_post = 64*1024;
+my $limit_max_post = 512*1024;
 
 my $known_methods = {
     GET => 1,
@@ -1891,7 +1891,7 @@ sub accept_connections {
 	    $self->{conn_count}++;
 	    $reqstate->{hdl} = AnyEvent::Handle->new(
 		fh => $clientfh,
-		rbuf_max => 64*1024,
+		rbuf_max => $limit_max_post + ($limit_max_headers * $limit_max_header_size),
 		timeout => $self->{timeout},
 		linger => 0, # avoid problems with ssh - really needed ?
 		on_eof   => sub {
-- 
2.39.5



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