Re: core(5)

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Jonny,

On 6/6/20 11:16 PM, Jonny Grant wrote:
> 
> 
> On 06/06/2020 20:39, Michael Kerrisk (man-pages) wrote:
>> On Sat, 6 Jun 2020 at 20:32, Jakub Wilk <jwilk@jwilk.net> wrote:
>>>
>>> * Jonny Grant <jg@jguk.org>, 2020-06-06, 16:45:
>>>>>> 3) Could i ask to clarify my understanding. For this "The binary
>>>>>> being executed by the process does not have read permission enabled."
>>>>>> -- is this when the binary permissions are changed after it starts
>>>>>> running?
>>>>> No, AFAICS the permission check is done when the process starts.
>>>> How can the process start if the binary file doesn't have read
>>>> permissions enabled?
>>>
>>> It's a bit weird, but the kernel doesn't mind:
>>>
>>>     $ cp /bin/ls .
>>>     $ chmod a-r ls
>>>     $ ./ls -l ls
>>>     --wx--x--x 1 jwilk jwilk 138856 Jun  6 20:22 ls
>>
>> And from core(5):
>>
>>         There are various circumstances in which a core dump file  is  not
>>         produced:
>>         ...
>>         *  The binary being executed by the process  does  not  have  read
>>            permission enabled.
>>
>> So, the binary can be executed, but not read, and will not do a core
>> dump (since that might be readable).
>>
>> Thanks,
>>
>> Michael
> 
> Hi Michael, Jakub,
> 
> It sounds like a good security feature. Could that be documented on the man page as the reason?
> 
> ie something like this:
> 
> *  The binary being executed by the process  does  not  have  read 
> permission enabled, therefore a core file would reveal information in
> a readable file, so it cannot be dumped.

Thanks for the suggested wording. I applied the patch below.

Thanks,

Michael

diff --git a/man5/core.5 b/man5/core.5
index 456366ea6..0d7b7b617 100644
--- a/man5/core.5
+++ b/man5/core.5
@@ -90,6 +90,10 @@ in
 .IP *
 The binary being executed by the process does not have read
 permission enabled.
+(This is a security measure to
+ensure that an executable whose contents are not readable
+does not produce a\(empossibly readable\(emcore dump containing
+an image of the executable.)
 .IP *
 The process is executing a set-user-ID (set-group-ID) program
 that is owned by a user (group) other than the real user (group)
-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/