Dict proxy client returning empty string instead of multiline string

[Pierre Jaury]

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.
> 
> 
>