linux-kernel - Re: [PATCH v2] kdb: Use strscpy with destination buffer size

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Date:   Thu, 18 Oct 2018 08:15:04 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     linux-kernel@...r.kernel.org,
        Jonathan Toppins <jtoppins@...hat.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH v2] kdb: Use strscpy with destination buffer size



On 10/04/2018 08:45 AM, Daniel Thompson wrote:
> On Thu, Oct 04, 2018 at 08:25:30AM -0400, Prarit Bhargava wrote:
>> On 10/02/2018 11:53 AM, Daniel Thompson wrote:
>>> On 20/09/2018 13:59, Prarit Bhargava wrote:
>>>> gcc 8.1.0 warns with:
>>>>
>>>> kernel/debug/kdb/kdb_support.c: In function ‘kallsyms_symbol_next’:
>>>> kernel/debug/kdb/kdb_support.c:239:4: warning: ‘strncpy’ specified bound
>>>> depends on the length of the source argument [-Wstringop-overflow=]
>>>>       strncpy(prefix_name, name, strlen(name)+1);
>>>>       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> kernel/debug/kdb/kdb_support.c:239:31: note: length computed here
>>>>
>>>> Use strscpy() with the destination buffer size, and use ellipses when
>>>> displaying truncated symbols.
>>>>
>>>> v2: Use strscpy()
>>>>
>>>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
>>>> Cc: Jonathan Toppins <jtoppins@...hat.com>
>>>> Cc: Jason Wessel <jason.wessel@...driver.com>
>>>> Cc: Daniel Thompson <daniel.thompson@...aro.org>
>>>> Cc: kgdb-bugreport@...ts.sourceforge.net
>>>> ---
>>>>   kernel/debug/kdb/kdb_io.c      | 15 +++++++++------
>>>>   kernel/debug/kdb/kdb_private.h |  2 +-
>>>>   kernel/debug/kdb/kdb_support.c | 10 +++++-----
>>>>   3 files changed, 15 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
>>>> index ed5d34925ad0..6a4b41484afe 100644
>>>> --- a/kernel/debug/kdb/kdb_io.c
>>>> +++ b/kernel/debug/kdb/kdb_io.c
>>>> @@ -216,7 +216,7 @@ static char *kdb_read(char *buffer, size_t bufsize)
>>>>       int count;
>>>>       int i;
>>>>       int diag, dtab_count;
>>>> -    int key;
>>>> +    int key, buf_size, ret;
>>>>           diag = kdbgetintenv("DTABCOUNT", &dtab_count);
>>>> @@ -336,9 +336,8 @@ static char *kdb_read(char *buffer, size_t bufsize)
>>>>           else
>>>>               p_tmp = tmpbuffer;
>>>>           len = strlen(p_tmp);
>>>> -        count = kallsyms_symbol_complete(p_tmp,
>>>> -                         sizeof(tmpbuffer) -
>>>> -                         (p_tmp - tmpbuffer));
>>>> +        buf_size = sizeof(tmpbuffer) - (p_tmp - tmpbuffer);
>>>> +        count = kallsyms_symbol_complete(p_tmp, buf_size);
>>>>           if (tab == 2 && count > 0) {
>>>>               kdb_printf("\n%d symbols are found.", count);
>>>>               if (count > dtab_count) {
>>>> @@ -350,9 +349,13 @@ static char *kdb_read(char *buffer, size_t bufsize)
>>>>               }
>>>>               kdb_printf("\n");
>>>>               for (i = 0; i < count; i++) {
>>>> -                if (WARN_ON(!kallsyms_symbol_next(p_tmp, i)))
>>>> +                ret = kallsyms_symbol_next(p_tmp, i, buf_size);
>>>> +                if (WARN_ON(!ret))
>>>>                       break;
>>> I'm getting confused by having two different branches on ret.
>>>
>>> Don't get a WARN_ON() when ret == -E2BIG?
>>>
>>>
>>
>> Should we WARN on a really long symbol?  I don't think we should as we're
>> handling that by truncating the output and adding ellipses below.
> 
> It's OK. You describe the behaviour I expect but I was misreading the
> code (not realizing that kallsyms_symbol_next() had not become a 0 on
> success success function).
> 
> However after reviewing the code (properly this time) I wonder if the
> WARN_ON() should be improved to match the return value for the function:
> 
>   WARN_ON(ret >= 0 && ret < len)
> 
> That said, checking the symbol length is pretty paranoid and getting
> close to nitpicking so I'll leave it up to you and, with or without, the
> change:
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>
> 
> 

I think we should leave it as is for now.  OOC was this patch picked up by
anyone's repo for staging for 4.20?

P.

> Daniel.
>