Re: PROBLEM: Data corruption when pasting large data to terminal

[Greg KH]

On Wed, Feb 15, 2012 at 07:50:58PM +0100, Egmont Koblinger wrote:
> Hi,
> 
> Short summary:  When pasting large amount of data (>4kB) to terminals,
> often the data gets mangled.
> 
> How to reproduce:
> Create a text file that contains this line about 100 times:
> a=(123456789123456789123456789123456789123456789123456789123456789)
> (also available at http://pastebin.com/LAH2bmaw for a while)
> and then copy-paste its entire contents in one step into a "bash" or
> "python" running in a graphical terminal.
> 
> Expected result: The interpreter correctly parses these lines and
> produces no visible result.
> Actual result: They complain about syntax error.
> Reproducibility: About 10% on my computer (2.6.38.8), reportedly 100%
> on friends' computers running 2.6.37 and 3.1.1.

Has this ever worked properly for you on older kernels?  How about 3.2?
3.3-rc3?  Having a "known good" point to work from here would be nice to
have.

I can reproduce this using bash, BUT, I can not reproduce it using vim
running in the same window bash was running in.

So, that implies that this is a userspace bug, not a kernel one,
otherwise the results would be the same both times, right?

> Why I believe this is a kernel bug:
> - Reproducible with any source of copy-pasting (e.g. various
> terminals, graphical editors, browsers).

Bugs are common when people start with the same original codebase :)

> - Reproducible with at least five different popular graphical terminal
> emulators where you paste into (xterm, gnome, kde, urxvt, putty).
> - Reproducble with at least two applications (bash, python).

Again, I can't duplicate this with vim in a terminal window, which rules
out the terminal, and points at bash, right?

> - stracing the terminal shows that it does indeed write the correct
> copy-paste buffer into /dev/ptmx, and all its writes return the full
> amount of bytes requested, i.e. no short write.

short writes are legal, but so many userspace programs don't handle them
properly.

> - stracing the application clearly shows that it does not receive all
> the desired characters from its stdin, some are simply missing, i.e. a
> read(0, "3", 1) = 1 is followed by a read(0, "\n", 1) = 1 (with a
> write() and some rt_sigprocmask()s in between), although the char '3'
> shouldn't be followed by a newline.

Perhaps the buffer is overflowing as the program isn't able to keep up
properly?  It's not an "endless" buffer, it can overflow if reads don't
keep up.

> - Not reproducible on MacOS.

That means nothing :)

> Additional informaiton:
> - On friends' computers the bug always happens from the offset 4163
> which is exactly the length of the first line (data immediately
> processed by the application) plus the magic 4095. The rest of that
> line, up to the next newline, is cut off.
> 
> - On my computer, the bug, if happens, always happens at an offset
> behind this one; moreover, there's a lone digit '3' appearing on the
> display on its own line exactly 4095 bytes before the syntax error.
> Here's a "screenshot" with "$ " being the bash prompt, and with my
> comments after "#":
> 
> $ a=(123456789123456789123456789123456789123456789123456789123456789)
> # repeated a few, varying number of times
> 3
> # <- notice this lone '3' on the display
> $ a=(123456789123456789123456789123456789123456789123456789123456789)
> # 60 times, that's 4080 bytes incl. newlines
> $ a=(123456789123
> > a=(123456789123456789123456789123456789123456789123456789123456789)
> bash: syntax error near unexpected token `('
> $ a=(123456789123456789123456789123456789123456789123456789123456789)
> # a few more times
> 
> - I couldn't reproduce with cat-like applications, I have a feeling
> perhaps the bug only occurs in raw terminal mode, but I'm really not
> sure about this.

That kind of proves the "there's a problem in the application you are
testing" theory, right?

> I'd be glad if you could find the time to look at this problem, it's
> quite unfortunate that I cannot safely copy-paste large amount of data
> into terminals.

Works for me, just use an editor to do that...

thanks,

greg k-h
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