[Crash-utility] log command and incorrect wrapping of the buffer

[Dave Anderson]

Alan Tyson wrote:
> Hi,
> 
> I've noticed on some systems that the crash log command shows output
> from the kernel log buffer which is not wrapped at the correct
> location.  All of the data from the buffer is present, but the display
> is not from oldest to newest.
> 
> Crash uses the pointer log_start to determine the "current" location in
> the log buffer.  This isn't correct.  log_start is the next location to
> be read by the syslog interface.  log_end is the pointer to the last
> byte written by printk (well, last byte written +1) and this is what
> should be used.
> 
> The quick fix is just to replace "log_start" with "log_end" in line 
> 3141.  The following tries to optimise the code a little and save
> a gdb call some of the time.
> 
> Thanks,
> 
> Alan Tyson, HP.

Hi Alan,

This works for me -- it tested fine on ~100 dumps, uncovering several
with the incorrect chronology.

Queued for the next release.

Thanks,
   Dave

FYI, the patch was munged -- all tabs were converted to spaces...


> 
> --- crash-4.0-4.7/kernel.c      2007-09-25 16:01:56.000000000 +0100
> +++ crash-4.0-4.7-at/kernel.c   2007-09-26 16:06:20.000000000 +0100
> @@ -3111,7 +3111,7 @@ void
> dump_log(int msg_level)
> {
>        int i;
> -       ulong log_buf, log_start, logged_chars;
> +       ulong log_buf, logged_chars;
>        char *buf;
>        char last;
>        ulong index;
> @@ -3138,13 +3138,16 @@ dump_log(int msg_level)
> 
>        buf = GETBUF(log_buf_len);
>        log_wrap = FALSE;
> -       get_symbol_data("log_start", sizeof(ulong), &log_start);
>        get_symbol_data("logged_chars", sizeof(ulong), &logged_chars);
>         readmem(log_buf, KVADDR, buf,
>                log_buf_len, "log_buf contents", FAULT_ON_ERROR);
> 
> -       log_start &= log_buf_len-1;
> -       index = (logged_chars < log_buf_len) ? 0 : log_start;
> +       if (logged_chars < log_buf_len) {
> +               index = 0;
> +       } else {
> +               get_symbol_data("log_end", sizeof(ulong), &index);
> +               index &= log_buf_len-1;
> +       }
> 
>        if ((logged_chars < log_buf_len) && (index == 0) && (buf[index] 
> == '<'))
>                loglevel = TRUE;
> 
> 
> 
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