Re: [PATCH v2 3/3] sideband: add defense against packets missing a band designator

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Jeff King <peff@peff.net>
Cc: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org, Jonathan Tan <jonathantanmy@google.com>
Subject: Re: [PATCH v2 3/3] sideband: add defense against packets missing a band designator
Date: Fri, 23 Oct 2020 07:36:25 +0200 (CEST)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.2010230732250.56@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <20201023084827.GE4012156@coredump.intra.peff.net>

Hi Peff,

On Fri, 23 Oct 2020, Jeff King wrote:

> On Mon, Oct 19, 2020 at 07:35:42PM +0000, Johannes Schindelin via GitGitGadget wrote:
>
> > diff --git a/pkt-line.c b/pkt-line.c
> > index 657a702927..f72048f623 100644
> > --- a/pkt-line.c
> > +++ b/pkt-line.c
> > @@ -461,8 +461,10 @@ int recv_sideband(const char *me, int in_stream, int out)
> >  	enum sideband_type sideband_type;
> >
> >  	while (1) {
> > -		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
> > -				  0);
> > +		int status = packet_read_with_status(in_stream, NULL, NULL, buf,
> > +						     LARGE_PACKET_MAX, &len, 0);
> > +		if (!len && status == PACKET_READ_NORMAL)
> > +			BUG("missing band designator");
> >  		if (!demultiplex_sideband(me, buf, len, 0, &scratch,
> >  					  &sideband_type))
>
> I also wonder if this status-check should be pushed down into
> demultiplex_sideband() by passing "status",

I tried that, but as mentioned in the commit message, fbd76cd450
(sideband: reverse its dependency on pkt-line, 2019-01-16) went out of its
way to _stop_ the code inside `demultiplex_sideband()` from relying on
anything in `pkt-line.h`. And that `PACKET_READ_NORMAL` and
`PACKET_READ_EOF` _is_ from `pkt-line.h`.

Ciao,
Dscho

> for two reasons:
>
>   1. So we don't have to repeat it (though it isn't that big)
>
>   2. The other half of this weirdness is that if we get an early EOF,
>      we'll hit the "missing sideband designator" die() message. But
>      that's not really what happened; we probably got a network hangup.
>      And we could distinguish that case by checking for status ==
>      PACKET_READ_EOF and provide a better message.
>
> Something like this (completely untested):
>
> diff --git a/sideband.c b/sideband.c
> index 0a60662fa6..6ad15ed581 100644
> --- a/sideband.c
> +++ b/sideband.c
> @@ -115,6 +115,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
>  #define DUMB_SUFFIX "        "
>
>  int demultiplex_sideband(const char *me, char *buf, int len,
> +			 enum packet_read_status status,
>  			 int die_on_error,
>  			 struct strbuf *scratch,
>  			 enum sideband_type *sideband_type)
> @@ -130,17 +131,29 @@ int demultiplex_sideband(const char *me, char *buf, int len,
>  			suffix = DUMB_SUFFIX;
>  	}
>
> -	if (len == 0) {
> -		*sideband_type = SIDEBAND_FLUSH;
> -		goto cleanup;
> -	}
> -	if (len < 1) {
> +	if (status == PACKET_READ_EOF) {
>  		strbuf_addf(scratch,
> -			    "%s%s: protocol error: no band designator",
> +			    "%s%s: protocol error: eof while reading packet",
>  			    scratch->len ? "\n" : "", me);
>  		*sideband_type = SIDEBAND_PROTOCOL_ERROR;
>  		goto cleanup;
>  	}
> +
> +	if (len < 0)
> +		BUG("negative length on non-eof packet read");
> +
> +	if (len == 0) {
> +		if (status == PACKET_READ_NORMAL) {
> +			strbuf_addf(scratch,
> +				    "%s%s protocol error: no band designator",
> +				    scratch->len ? "\n" : "", me);
> +			*sideband_type = SIDEBAND_PROTOCOL_ERROR;
> +		} else {
> +			*sideband_type = SIDEBAND_FLUSH;
> +		}
> +		goto cleanup;
> +	}
> +
>  	band = buf[0] & 0xff;
>  	buf[len] = '\0';
>  	len--;
>