Re: RFR[9u-dev]: 8151442: jstack doesn't close quotation marks properly with threads' name greater than 1996 characters

[David Holmes]

On 18/03/2016 11:28 PM, Dmitry Samersoff wrote:
David,

Ignoring Dmitry's issue it still seems simpler/cleaner to just add the
desired precision value to the %s than to split into two print
statements. Or bite the bullet now and make the length immaterial by
breaking the name into chunks. It's as easy to fix as to write the
RFE :)

For this particular case the simplest solution is to use print_raw:

print_raw("\""\"); print_raw(get_thread_name()); print_raw("\""\");

I still see %.Ns as the simplest solution - but whatever.


But as soon as print() finally ends up with:

const int written = vsnprintf(buffer, buflen, format, ap);
...
DEBUG_ONLY(warning("increase O_BUFLEN in ostream.hpp -- output truncated");)

Complete fix should be something like:

int desired_size = vsnprintf(NULL, 0, format, ap);
if (desired_size > O_BUFLEN) {
     malloc
     ....
}

but it has performance penalty, so we should use some tricks to cover
most common %s/%d/%p cases.

So you want to remove the internal limitation instead of have the 
clients deal with it? Not sure it is worth the effort - IIRC that code 
can be used at sensitive times hence the simple approach to buffer 
management.

David

-Dmitry




On 2016-03-18 15:51, David Holmes wrote:
On 18/03/2016 10:03 PM, Cheleswer Sahu wrote:
Hi David,

-----Original Message-----
From: David Holmes
Sent: Friday, March 18, 2016 2:42 PM
To: Cheleswer Sahu; hotspot-runtime-...@openjdk.java.net;
serviceability-dev@openjdk.java.net
Subject: Re: RFR[9u-dev]: 8151442: jstack doesn't close quotation
marks properly with threads' name greater than 1996 characters

On 18/03/2016 5:54 PM, Cheleswer Sahu wrote:
Hi,

Please review the code changes for
https://bugs.openjdk.java.net/browse/JDK-8151442.

Webrev Link: http://cr.openjdk.java.net/~csahu/8151442/

Bug Brief:

In jstack thread dumps , thread name greater than 1996 characters
doesn't close quotation marks properly.

Anyone giving a thread a name that long deserves to get a core dump! ;-)

Problem Identified:

Jstack is using below code to print thread name

src/share/vm/runtime/thread.cpp

void JavaThread::print_on(outputStream *st) const {

     st->print("\"%s\" ", get_thread_name());

Here "st->print()"  internally uses max buffer length as O_BUFLEN
(2000).

void outputStream::do_vsnprintf_and_write_with_automatic_buffer(const
char* format, va_list ap, bool add_cr) {

     char buffer[O_BUFLEN];

do_vsnprintf_and_write_with_automatic_buffer() finally calls
    "vsnprintf()"  which truncates the anything greater than the max
size(2000). In this case thread's name(> 1996) along with quotation
marks (2)

plus one terminating character exceeds the  max buffer size (2000),
therefore the closing quotation  marks gets truncated.

Solution:

Split the  "st->print("\"%s\" ", get_thread_name())" in two statements

1.st->print("\"%s", get_thread_name());

2.st->print("\" ");

This will ensure presence of closing quotation mark always.

Can't we just limit the number of characters read by %s?

Yes we can do it, but just one thing which I think of is, if the
truncation of output inside  output stream issue get resolves as
Dmritry suggested or O_BUFFLEN size gets increased in any  future fix
then we have to again make changes in this code, as limiting the no.
of character read by %s will give truncated output . In such case
present fix will have no effect.

Ignoring Dmitry's issue it still seems simpler/cleaner to just add the
desired precision value to the %s than to split into two print
statements. Or bite the bullet now and make the length immaterial by
breaking the name into chunks. It's as easy to fix as to write the RFE :)

David

David

Regards,

Cheleswer