[Bug tapsets/19489] New: printing array from memory

[Bug tapsets/19489] New: printing array from memory

[josch@mister-muffin.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=19489

            Bug ID: 19489
           Summary: printing array from memory
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: josch@mister-muffin.de
  Target Milestone: ---

Hi,

consider the following C program:

    #include <unistd.h>

    int main()
    {
        char buffer[7] = {'f', 'o', 'o', 'b', 'a', 'r', '\n'};
        write(1, buffer, 7);
        char buffer2[2] = {'\0', '\n'};
        write(1, buffer2, 2);
        return 0;
    }

and the following systemtap script:

    probe syscall.write {
        if (!target_set_pid(pid())) next;
        printf("%s %s\n", name, argstr)
        printf("%*M\n", $count, $buf);
    }

execute like so:

    $ sudo stap -v stdoutbuf.stp -o out -c "./a.out"

I'd expect the file `out` to contain the following content:

    write 1, "foobar\n", 7
    666f6f6261720a
    write 1, "\000\n", 2
    000a

Instead I get

    write 1, "foobar\n", 7
         66
    write 1, "", 2
    00

Notice the two problems:

 1. apparently the argstr cannot handle the zero byte and instead of octal
encoding it, it just truncates the string.
 2. printf("%*M") is not printing the full string

System information:

Operating system: Debian unstable
Kernel version: 4.4.0-rc8-amd64
Architecture: amd64
Systemtap version: 2.9/0.163, Debian version 2.9-2
gcc version: 5.2.1 20151028 (Debian 5.2.1-23)

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[Bug tapsets/19489] printing array from memory

[fche at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19489

Frank Ch. Eigler <fche at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fche at redhat dot com

--- Comment #1 from Frank Ch. Eigler <fche at redhat dot com> ---
Thanks for your report.

The core of the problem appears to be our use of user_string_n2_quoted in the
syscall.write alias (see .../tapset/linux/syscalls2.stp), specifically the
interpretation of the "n".  The function interprets it as a maximum, not an
exact buffer length (see [man function::user_string_n2_quoted]'s mention of C
strings).

We could revise the behaviour of that function to treat its input as a buffer
of definite size, if most or all other uses of that function would benefit.

In the interim, you can use

    printf("%.*M", $count, $buf)

(note the    ^    dot!)

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[Bug tapsets/19489] printing array from memory

[csanting at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19489

Cody Santing <csanting at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |csanting at sourceware dot org
           Assignee|systemtap at sourceware dot org    |csanting at sourceware dot org

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