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Tag: Linux (Page 1 of 10)

Use network analyzer to learn SSH session establishment

The establishment of SSH session consists of 2 parts: build up the encryption channel and authenticate user. To understand the whole flow better, I usetcpdump/Wireshark to capture and analyze the packets. Server is OpenBSD 6.1 and client is ArchLinux. The tcpdump command is like this:

sudo tcpdump -A -s 0 'net 192.168.38.176' -i enp7s0f0 -w capture.pcap

(1) Connect server first time:

1

The captured packets:

C1

We can see the client/server negotiated SSH version first, then exchanged public key to generate secret key. The server issued “New Keys” message, and waited for client to answer.

(2) Accept server’s public key but not input password:

2

The captured packets:

C2

The first packet should be client acknowledged server’s “New Keys” message, then there are some interactions. Now the encryption channel is set up.

(3) Enter password and authenticate user:

3

The captured packets:

C3

These packets are all encrypted data. If user’s password is correct, the whole SSH session will be ready, and you can administrator server now.

Reference:
Understanding the SSH Encryption and Connection Process.

Configure Thunderbird to send patch friendly

Recently, I have tried outlook and web gmail to send patches to OpenBSD, but neither of them is an ideal choice. For example, the original patch is:

Index: dmesg.c
===================================================================
RCS file: /cvs/src/sbin/dmesg/dmesg.c,v
retrieving revision 1.29
diff -u -p -r1.29 dmesg.c
--- dmesg.c     1 Sep 2017 07:31:45 -0000       1.29
+++ dmesg.c     4 Sep 2017 08:55:50 -0000
@@ -65,12 +65,12 @@ main(int argc, char *argv[])
        int ch, newl, skip, i;
        char *p;
        struct msgbuf cur;
-       char *memf, *nlistf, *bufdata = NULL;
+       char *memf = NULL, *nlistf = NULL, *bufdata = NULL;
        char *allocated = NULL;
        int startupmsgs = 0;
        char buf[5];

-       memf = nlistf = NULL;
+       memset(&cur, 0, sizeof(cur));
        while ((ch = getopt(argc, argv, "sM:N:")) != -1)
                switch(ch) {
                case 's':

The actual effect of sending patch using above tools:

Index: dmesg.c
===================================================================
RCS file: /cvs/src/sbin/dmesg/dmesg.c,v
retrieving revision 1.29
diff -u -p -r1.29 dmesg.c
--- dmesg.c 1 Sep 2017 07:31:45 -0000   1.29
+++ dmesg.c 4 Sep 2017 08:55:50 -0000
@@ -65,12 +65,12 @@ main(int argc, char *argv[])
    int ch, newl, skip, i;
    char *p;
    struct msgbuf cur;
-   char *memf, *nlistf, *bufdata = NULL;
+   char *memf = NULL, *nlistf = NULL, *bufdata = NULL;
    char *allocated = NULL;
    int startupmsgs = 0;
    char buf[5];

-   memf = nlistf = NULL;
+   memset(&cur, 0, sizeof(cur));
    while ((ch = getopt(argc, argv, "sM:N:")) != -1)
    switch(ch) {
    case 's':

We can see the spaces in the left part of code are lost. After referring Completely plain email and Thunderbird (GUI), I decide to useThunderbird as the email client, but need further customization:

(1) All sent mails are plain text: “Account Settings” -> “Composition & Addressing“, untick “Compose message in HTML format“:

1

Or before sending mail: “Options” -> “Delivery Format” -> “Plain Text Only“.

(2) “Tools” -> “Options” -> “Advanced” -> “Config Editor...“:

2

Set mailnews.send_plaintext_flowed to false:

3

Now the patch format is correct.

Build gRPC on ArchLinux

Today, I followed Build from Source to compile gRPC on ArchLinux:

 $ git clone -b $(curl -L https://grpc.io/release) https://github.com/grpc/grpc
 $ cd grpc
 $ git submodule update --init
 $ make

Current gRPC‘s release version is v1.4.x:

$ curl -L https://grpc.io/release
v1.4.x

The build flow will generate the errors like this:

......
src/core/lib/support/murmur_hash.c: In function ‘gpr_murmur_hash3’:
src/core/lib/support/murmur_hash.c:79:10: error: this statement may fall through [-Werror=implicit-fallthrough=]
       k1 ^= ((uint32_t)tail[2]) << 16;
       ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/core/lib/support/murmur_hash.c:80:5: note: here
     case 2:
     ^~~~
src/core/lib/support/murmur_hash.c:81:10: error: this statement may fall through [-Werror=implicit-fallthrough=]
       k1 ^= ((uint32_t)tail[1]) << 8;
       ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/core/lib/support/murmur_hash.c:82:5: note: here
     case 1:
     ^~~~
cc1: all warnings being treated as errors
......

After referring Fix warnings with GCC 7, I finally make the compilation successful. To facilitate others to build gRPC v1.4.x source code on ArchLinux, I create a patch, and hope it can help others.

P.S.
(1) You should fallback to OpenSSL 1.0. Please refer here:

PKG_CONFIG_PATH=/usr/lib/openssl-1.0/pkgconfig make

Otherwise you may encounter following errors:

src/core/tsi/ssl_transport_security.c: In function ‘tsi_create_ssl_client_handshaker_factory’:
src/core/tsi/ssl_transport_security.c:1281:3: error: ‘TLSv1_2_method’ is deprecated [-Werror=deprecated-declarations]
   ssl_context = SSL_CTX_new(TLSv1_2_method());
   ^~~~~~~~~~~
In file included from /usr/include/openssl/ct.h:13:0,
                 from /usr/include/openssl/ssl.h:61,
                 from src/core/tsi/ssl_transport_security.c:45:
/usr/include/openssl/ssl.h:1624:1: note: declared here
 DEPRECATEDIN_1_1_0(__owur const SSL_METHOD *TLSv1_2_method(void)) /* TLSv1.2 */
 ^
src/core/tsi/ssl_transport_security.c: In function ‘tsi_create_ssl_server_handshaker_factory_ex’:
src/core/tsi/ssl_transport_security.c:1389:7: error: ‘TLSv1_2_method’ is deprecated [-Werror=deprecated-declarations]
       impl->ssl_contexts[i] = SSL_CTX_new(TLSv1_2_method());
       ^~~~
In file included from /usr/include/openssl/ct.h:13:0,
                 from /usr/include/openssl/ssl.h:61,
                 from src/core/tsi/ssl_transport_security.c:45:
/usr/include/openssl/ssl.h:1624:1: note: declared here
 DEPRECATEDIN_1_1_0(__owur const SSL_METHOD *TLSv1_2_method(void)) /* TLSv1.2 */
 ^
At top level:
src/core/tsi/ssl_transport_security.c:118:22: error: ‘openssl_thread_id_cb’ defined but not used [-Werror=unused-functio ]
 static unsigned long openssl_thread_id_cb(void) {
                      ^~~~~~~~~~~~~~~~~~~~
src/core/tsi/ssl_transport_security.c:110:13: error: ‘openssl_locking_cb’ defined but not used [-Werror=unused-function]
 static void openssl_locking_cb(int mode, int type, const char *file, int line) {
             ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

(2) You may need to change installation directory from /usr/local to /usr:

make prefix=/usr install

This lets you process pkg-config path easily.

Parse BPF_ARRAY macro in bcc

BPF_ARRAY is a very common macro used in bcc scripts. To analyze it, I put all BPF_ARRAY related macros in an example file:

// Changes to the macro require changes in BFrontendAction classes
#define BPF_F_TABLE(_table_type, _key_type, _leaf_type, _name, _max_entries, _flags) \
struct _name##_table_t { \
  _key_type key; \
  _leaf_type leaf; \
  _leaf_type * (*lookup) (_key_type *); \
  _leaf_type * (*lookup_or_init) (_key_type *, _leaf_type *); \
  int (*update) (_key_type *, _leaf_type *); \
  int (*insert) (_key_type *, _leaf_type *); \
  int (*delete) (_key_type *); \
  void (*call) (void *, int index); \
  void (*increment) (_key_type); \
  int (*get_stackid) (void *, u64); \
  _leaf_type data[_max_entries]; \
  int flags; \
}; \
__attribute__((section("maps/" _table_type))) \
struct _name##_table_t _name = { .flags = (_flags) }

#define BPF_TABLE(_table_type, _key_type, _leaf_type, _name, _max_entries) \
BPF_F_TABLE(_table_type, _key_type, _leaf_type, _name, _max_entries, 0);

#define BPF_ARRAY1(_name) \
  BPF_TABLE("array", int, u64, _name, 10240)
#define BPF_ARRAY2(_name, _leaf_type) \
  BPF_TABLE("array", int, _leaf_type, _name, 10240)
#define BPF_ARRAY3(_name, _leaf_type, _size) \
  BPF_TABLE("array", int, _leaf_type, _name, _size)

// helper for default-variable macro function
#define BPF_ARRAYX(_1, _2, _3, NAME, ...) NAME

// Define an array function, some arguments optional
// BPF_ARRAY(name, leaf_type=u64, size=10240)
#define BPF_ARRAY(...) \
  BPF_ARRAYX(__VA_ARGS__, BPF_ARRAY3, BPF_ARRAY2, BPF_ARRAY1)(__VA_ARGS__)

enum stat_types {
  S_COUNT = 1,
  S_MAXSTAT
};

void main(void)
{
  BPF_ARRAY(stats, u64, S_MAXSTAT + 1); 
}

Use gcc -E to preprocess it (I have formatted code to make it clear):

# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "test.c"
# 38 "test.c"
enum stat_types {
    S_COUNT = 1,
    S_MAXSTAT
};

void main(void)
{
    struct stats_table_t { 
      int key; 
      u64 leaf; 
      u64 * (*lookup) (int *); 
      u64 * (*lookup_or_init) (int *, u64 *); 
      int (*update) (int *, u64 *); 
      int (*insert) (int *, u64 *); 
      int (*delete) (int *);
      void (*call) (void *, int index); 
      void (*increment) (int); 
      int (*get_stackid) (void *, u64); 
      u64 data[S_MAXSTAT + 1]; 
      int flags; 
    }; 
    __attribute__((section("maps/" "array"))) struct stats_table_t stats = { .flags = (0) };;
}

Let’s analyze how the final result is got:

(1)

BPF_ARRAY(stats, u64, S_MAXSTAT + 1)    

will be expanded to (__VA_ARGS__ will be replaced by all arguments):

BPF_ARRAYX(stats, u64, S_MAXSTAT + 1, BPF_ARRAY3, BPF_ARRAY2, BPF_ARRAY1)(stats, u64, S_MAXSTAT + 1)

(2) According to:

// helper for default-variable macro function
#define BPF_ARRAYX(_1, _2, _3, NAME, ...) NAME

BPF_ARRAYX(stats, u64, S_MAXSTAT + 1, BPF_ARRAY3, BPF_ARRAY2, BPF_ARRAY1)(stats, u64, S_MAXSTAT + 1) will be replaced by following code:

BPF_ARRAY3(stats, u64, S_MAXSTAT + 1)

We can compared it to BPF_ARRAY1 (both _leaf_type and size have default values) and BPF_ARRAY2 (only size have default value.).

(3) Convert BPF_ARRAY3 to final BPF_F_TABLE is more straightforward, so I won’t drill that down.

Use the same method you can analyze other macros, such as BPF_HASH. Hope this small post can give you a tip!

The subtlety of building ANTLR C runtime library

If you want to use ANTLR C runtime library, you should pay attention that it will be compiled to 32-bit library by default even in 64-bit platform:

# ./configure
......
# make
make  all-am
make[1]: Entering directory '/root/Project/libantlr3c-3.4'
/bin/sh ./libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -Iinclude    -m32  -O2  -Wall -MT antlr3baserecognizer.lo -MD -MP -MF .deps/antlr3baserecognizer.Tpo -c -o antlr3baserecognizer.lo `test -f 'src/antlr3baserecognizer.c' || echo './'`src/antlr3baserecognizer.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -Iinclude -m32 -O2 -Wall -MT antlr3baserecognizer.lo -MD -MP -MF .deps/antlr3baserecognizer.Tpo -c src/antlr3baserecognizer.c  -fPIC -DPIC -o .libs/antlr3baserecognizer.o
In file included from /usr/include/features.h:434:0,
                 from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdio.h:28,
                 from include/antlr3defs.h:248,
                 from include/antlr3baserecognizer.h:39,
                 from src/antlr3baserecognizer.c:9:
/usr/include/gnu/stubs.h:7:11: fatal error: gnu/stubs-32.h: No such file or directory
 # include <gnu/stubs-32.h>
           ^~~~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:449: antlr3baserecognizer.lo] Error 1
make[1]: Leaving directory '/root/Project/libantlr3c-3.4'
make: *** [Makefile:308: all] Error 2

In order to build 64-bit library, you should specify --enable-64bit option during configure stage:

./configure --enable-64bit

Then it will be generated to 64-bit library.

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