Dict proxy client returning empty string instead of multiline string
Pierre Jaury
pierre at jaury.eu
Mon Oct 17 13:14:55 UTC 2016
Okay, it seems to me that the bug is due to "t_str_tabunescape" using
the unsafe datastack ("t_strdup_noconst") while the string is actually
returned in an async callback.
Before it is handled by "client_dict_lookup", "client_dict_wait"
actually fires some IO loops that are more than likely to call "t_pop"
and free/flush the result string (I checked, it does call "t_pop" a
couple times indeed). Maybe it would be safer to use a standard
datastack pool when unescaping a string in that context.
I could work on a patch that would:
- add an optional "pool" attribute to the "client_dict_cmd" structure,
- pass the pool to the async lookup function,
- use the pool when escaping strings that should survive the callback
chain
What do you think?
Regards,
kaiyou
On 10/17/2016 09:52 AM, Pierre Jaury wrote:
> While trying to isolate properly and reproduce, I was able to trigger
> the same bug with the following code:
>
> struct dict *dict;
> char* dict_uri = "proxy::sieve";
> char* key = "priv/key";
> char* username = "admin at domain.tld";
> char* value, error;
>
> dict_drivers_register_builtin();
> dict_init(dict_uri, DICT_DATA_TYPE_STRING, username,
> doveadm_settings->base_dir, &dict, &error);
> dict_lookup(dict, pool_datastack_create(), key, &value);
> printf(">%s\n", value); // outputs an empty string
> dict_deinit(&dict);
>
> I trimmed it to the bare minimal string manipulation functions involved
> but cannot reproduce in that case:
>
> pool_t pool = pool_datastack_create();
>
> char* s1 = "test\001n\001rtest";
> char* s2 = t_str_tabunescape(s1);
> char* s3 = p_strdup(pool, s2);
>
> printf("1>%s\n", s1);
> printf("2>%s\n", s2);
> printf("3>%s\n", s3); // all three output the string with NL and CR
>
> Maybe I am missing a function call in the process or maybe the issue is
> related to the way unescaping is performed in the async callback
> function in "dict-client.c", or maybe even some other edge case.
>
> Finally, I was able to run the first snippet without bug by removing the
> string duplication in "t_str_tabunescape" (which I realize is not a
> proper solution), or by explicitely using the following pool:
>
> return str_tabunescape(p_strdup(pool_datastack_create(), str));
>
>
> Hope this helps.
>
> kaiyou
>
>
> On 10/17/2016 07:51 AM, Aki Tuomi wrote:
>> Hi!
>>
>> This does sound like a bug, we'll have look.
>>
>> Aki
>>
>>
>> On 17.10.2016 01:26, Pierre Jaury wrote:
>>> I dived a little bit further into the rabbit hole, up to the point where
>>> debugging has become unpracticle but I still haven't found the root
>>> cause for sure.
>>>
>>> I read most of the code for "p_strdup" based on datastack memory pools
>>> (which are used for dictionary lookups both with doveadm and by extdata)
>>> and it seems ok. Still, after "t_malloc_real" is called in "t_malloc0",
>>> the allocated buffer has the same address as the source string.
>>>
>>> The only sensible explanation I can come up with is that during
>>> unescaping, strings are not allocated properly, leading to the memory
>>> pool reusing the string address and zeroing it in the process before the
>>> string copy operation.
>>>
>>> I will follow on this path tomorrow, any lead is more than welcome.
>>>
>>> kaiyou.
>>>
>>> On 10/16/2016 11:16 PM, Pierre Jaury wrote:
>>>> Hello,
>>>>
>>>> I am using a dict proxy for my sieve extdata plugin to access some
>>>> fields from an SQLite database (autoreply text and other
>>>> database-configured items).
>>>>
>>>> All tests are performed against version 2.2.25.
>>>>
>>>> $ dovecot --version
>>>> 2.2.25 (7be1766)
>>>>
>>>> My configuration looks like:
>>>>
>>>> dict {
>>>> sieve = sqlite:/etc/dovecot/pigeonhole-sieve.dict
>>>> }
>>>>
>>>> [...]
>>>> sieve_extdata_dict_uri = proxy::sieve
>>>>
>>>> I am able to read pretty much any attribute without any issue, except
>>>> when the value contains a special character like "\r" or "\n". By using
>>>> the doveadm dict client, I narrowed it down to the dictionary management
>>>> part (either server, protocol or client).
>>>>
>>>> I was suspecting escaping functions from "lib/strescape.c" (mostly
>>>> str_tabescape and its counterpart, used by "lib-dict/client.c"), so I
>>>> monitored socket communications. It seems that escaping is done properly
>>>> on the server and the socket is not an issue either.
>>>>
>>>> The following strace dump results from running "doveadm dict get"
>>>> against the dict socket:
>>>>
>>>> connect(8, {sa_family=AF_UNIX, sun_path="..."}, 110) = 0
>>>> fstat(8, {st_mode=S_IFSOCK|0777, st_size=0, ...}) = 0
>>>> [...]
>>>> write(8, "H2\t0\t0\tadmin at domain.tld\tsieve\n", 30) = 30
>>>> [...]
>>>> read(8, "Otest\1r\1ntest\n", 8192) = 14
>>>>
>>>> Indeed "\1r" and "\1n" are the escape sequences used by
>>>> "lib/strescape.c". I went deeped and debugged the call to "dict_lookup"
>>>> performed by doveadm. Indeed the client gets the proper string from the
>>>> socket and to my surprise, it is properly unescaped.
>>>>
>>>> Then, in "client_dict_lookup" ("lib-dict/dict-client.c"), the call to
>>>> "p_strdup" returns an empty string (null byte set at the target address).
>>>>
>>>> Before the call to the dict "->lookup" attribute (client_dict_lookup):
>>>>
>>>> RAX: 0x7ffff73a37c0 (push r14)
>>>> RBX: 0x6831b8 ("priv/reply_body")
>>>> RCX: 0x7fffffffe240 --> 0x682a60 --> 0x6831b8 ("priv/reply_body")
>>>> RDX: 0x6831b8 ("priv/reply_body")
>>>> RSI: 0x683288 --> 0x7ffff7653120 --> 0x7ffff73ea620 ([...])
>>>> RDI: 0x690ad0 --> 0x7ffff7400713 --> 0x75250079786f7270 ('proxy')
>>>>
>>>> 0x7ffff73a1f10 <dict_lookup+32>: mov rcx,r11 (value_r)
>>>> 0x7ffff73a1f13 <dict_lookup+35>: mov rdx,r8 (key)
>>>> 0x7ffff73a1f16 <dict_lookup+38>: mov rsi,r10 (pool)
>>>> 0x7ffff73a1f19 <dict_lookup+41>: mov rdi,r9 (dict)
>>>> 0x7ffff73a1f1c <dict_lookup+44>: add rsp,0x8
>>>> => 0x7ffff73a1f20 <dict_lookup+48>: jmp rax
>>>>
>>>> Before the call to p_strdup in "client_dict_lookup":
>>>>
>>>> RSI: 0x6832d8 ("test\r\ntest") (lookup.result.value)
>>>> RDI: 0x683288 --> 0x7ffff7653120 --> [...] (pool)
>>>> RAX: 0x0 (result)
>>>>
>>>> 0x7ffff73a384f: nop
>>>> 0x7ffff73a3850: mov rsi,QWORD PTR [rsp+0x8]
>>>> 0x7ffff73a3855: mov rdi,r14
>>>> => 0x7ffff73a3858: call 0x7ffff736d3c0 <p_strdup at plt>
>>>> 0x7ffff73a385d: mov QWORD PTR [r13+0x0],rax
>>>> 0x7ffff73a3861: mov rsi,QWORD PTR [rsp+0x18]
>>>> 0x7ffff73a3866: xor rsi,QWORD PTR fs:0x28
>>>> 0x7ffff73a386f: mov eax,ebx
>>>>
>>>> After the call:
>>>>
>>>> 0x7ffff73a3850: mov rsi,QWORD PTR [rsp+0x8]
>>>> 0x7ffff73a3855: mov rdi,r14
>>>> 0x7ffff73a3858: call 0x7ffff736d3c0 <p_strdup at plt>
>>>> => 0x7ffff73a385d: mov QWORD PTR [r13+0x0],rax
>>>> 0x7ffff73a3861: mov rsi,QWORD PTR [rsp+0x18]
>>>> 0x7ffff73a3866: xor rsi,QWORD PTR fs:0x28
>>>> 0x7ffff73a386f: mov eax,ebx
>>>> 0x7ffff73a3871: jne 0x7ffff73a38da
>>>>
>>>> RSI: 0x0
>>>> RDI: 0x6832d8 --> 0x0
>>>> RAX: 0x6832d8 --> 0x0 (result)
>>>>
>>>> It is worth noting that I can reproduce the exact same execution flow
>>>> with a non-multiline result string (lookup.result.value) that is
>>>> properly copied by "p_strdup" and returned in RAX, then displayed by
>>>> doveadm.
>>>>
>>>> I am not familiar with the pooling mechanism hidden behind the call to
>>>> p_strdump and not quite sure why this behaviour is emerging. Maybe I am
>>>> even miles away from an understanding of the issue here, but it sounds
>>>> to me like something is wrong in the way "p_strdup" performs the copy.
>>>>
>>>> Hope this helps,
>>>> kaiyou.
>>>>
>>>>
>>>>
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