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Re: bash sockets: printf \x0a does TCP fragmentation


From: Greg Wooledge
Subject: Re: bash sockets: printf \x0a does TCP fragmentation
Date: Tue, 25 Sep 2018 08:25:41 -0400
User-agent: NeoMutt/20170113 (1.7.2)

On Tue, Sep 25, 2018 at 05:15:02AM -0700, L A Walsh wrote:
> On 9/24/2018 6:05 AM, Greg Wooledge wrote:
> > On Sat, Sep 22, 2018 at 11:50:17AM +0200, address@hidden wrote:
> > > On 9/22/18 7:30 AM, Bob Proulx wrote:
> > > > address@hidden wrote:
> > > > > printf -- "$data" >&5 2>/dev/null
> > > > What happens if $data contains % format strings?  What happens if the
> > > > format contains a sequence such as \c?  This looks problematic.  This
> > > > is not a safe programming proctice.
> > 
> > Looking ONLY at this one line, there is an obvious bug, which Bob has
> > pointed out.  It should be
> > 
> > printf %s "$data" >&5 2>/dev/null
> ----
>    This brings to mind a consideration:
> As %s says to print a string of data (presumably not
> including a NUL byte), then what happens if "$data" is
> a paragraph of text with embedded newlines.  In that case,
> it sounds like bash might break apart the single printf
> output into smaller packets rather than transmitting the
> entirety of "$data" in 1 write (presuming it is less than
> the maximum data size for a network packet).

Yes, I'm sure it does.  In fact, bash's printf and echo builtins are
already known to use multiple calls to write() even when sockets and
newlines are not involved.

For example (from <https://mywiki.wooledge.org/BashPitfalls#pf51>):

$ perl -e 'print "a"x2000, "\n"' > foo
$ strace -e write bash -c 'read -r foo < foo; echo "$foo"' >/dev/null
write(1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 1008) = 1008
write(1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 993) = 993
+++ exited with 0 +++

There is no guarantee that the entire payload will be sent with a single
write() command.  If you need that kind of low-level control, bash is
not the right language for this project.



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