Re: [PATCH v4] coredump: Add /proc/<pid>/coredump_pre_exit for pre-exit before dumping

["David Hildenbrand (Arm)" <david@kernel.org>]

>> +
>>  #define F_DUPFD		0	/* dup */
>>  #define F_GETFD		1	/* get close_on_exec */
>>  #define F_SETFD		2	/* set/clear close_on_exec */
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index a679b2448234..84f1ee7f32cf 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1030,6 +1030,18 @@ static int __init coredump_filter_setup(char *s)
>>  
>>  __setup("coredump_filter=", coredump_filter_setup);
>>  
>> +static unsigned long default_dump_pre_exit;
>> +
>> +static int __init coredump_pre_exit_setup(char *s)
>> +{
>> +	default_dump_pre_exit =
>> +		(simple_strtoul(s, NULL, 0) << MMF_DUMP_PRE_EXIT_SHIFT) &
>> +		MMF_DUMP_PRE_EXIT_MASK;
>> +	return 1;
>> +}
>> +
>> +__setup("coredump_pre_exit=", coredump_pre_exit_setup);
> 
> This makes no sense. I think you really need to sit down and think about
> a design for this that doesn't introduce state machinery for boot, mm,
> and the VFS in one shot to solve a fringe problem...

Staring at exit_mmap_mapped_shared(), ... this looks rather hacky ("let's fake
munmap and set some magical flags").

We're essentially saying "we don't want (pretty much) anything that's MAP_SHARED
in the coredump". And for some reason someone should configure that, that's a
rather weird toggle tbh.

And the granularity ("file-backed shared memory") is completely odd.


Aren't there other ways we could optimize this internally?

Like, if we know that a process is dead and cannot run anymore, downgrade writes
to reads (and make sure we block GUP write attempts accordingly), or would that
also not be sufficient?


Another thought:

fs/coredump.c calls get_dump_page().

get_dump_page() will not fault in any memory. So if a page is not in the page
tables at the time of the dump, it will not get included in the coredump. Which
means, that whether most non-anonymous memory will be included in a coredump is
already like playing the lottery.

This is true for MAP_SHARED file mappings and MAP_PRIVATE file mappings without
private modifications.

Which makes me wonder: How much is tooling relying on file-backed pages to end
up in a coredump?

-- 
Cheers,

David