Re: Dict proxy client returning empty string instead of multiline string

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@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 : mov rcx,r11 (value_r) 0x7ffff73a1f13 : mov rdx,r8 (key) 0x7ffff73a1f16 : mov rsi,r10 (pool) 0x7ffff73a1f19 : mov rdi,r9 (dict) 0x7ffff73a1f1c : add rsp,0x8 => 0x7ffff73a1f20 : 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@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@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.