内核中dump_stack的实现原理 —— 内核函数printk的实现

Posted 2022-03-02 pengdonglin137

在内核中使用dump_stack的时候可以看到如下用法：

static inline void print_ip_sym(unsigned long ip)

    printk("[<%px>] %pS\\n", (void *) ip, (void *) ip);

 

然后我们就可以看到类似如下的内核log：

 

可以看到，上面不光输出了运行地址，还把对应的函数名以及偏移地址都列了出来，内核是如何做到的呢？

在调用printk的时候：

 

printk --> vprintk_default --> vprintk_emit --> vprintk_store --> vscnprintf --> vsnprintf --> pointer

 

上面的转换就是在pointer中完成的：

  1 /*
  2  * Show a ‘%p‘ thing.  A kernel extension is that the ‘%p‘ is followed
  3  * by an extra set of alphanumeric characters that are extended format
  4  * specifiers.
  5  *
  6  * Please update scripts/checkpatch.pl when adding/removing conversion
  7  * characters.  (Search for "check for vsprintf extension").
  8  *
  9  * Right now we handle:
 10  *
 11  * - ‘F‘ For symbolic function descriptor pointers with offset
 12  * - ‘f‘ For simple symbolic function names without offset
 13  * - ‘S‘ For symbolic direct pointers with offset
 14  * - ‘s‘ For symbolic direct pointers without offset
 15  * - ‘[FfSs]R‘ as above with __builtin_extract_return_addr() translation
 16  * - ‘B‘ For backtraced symbolic direct pointers with offset
 17  * - ‘R‘ For decoded struct resource, e.g., [mem 0x0-0x1f 64bit pref]
 18  * - ‘r‘ For raw struct resource, e.g., [mem 0x0-0x1f flags 0x201]
 19  * - ‘b[l]‘ For a bitmap, the number of bits is determined by the field
 20  *       width which must be explicitly specified either as part of the
 21  *       format string ‘%32b[l]‘ or through ‘%*b[l]‘, [l] selects
 22  *       range-list format instead of hex format
 23  * - ‘M‘ For a 6-byte MAC address, it prints the address in the
 24  *       usual colon-separated hex notation
 25  * - ‘m‘ For a 6-byte MAC address, it prints the hex address without colons
 26  * - ‘MF‘ For a 6-byte MAC FDDI address, it prints the address
 27  *       with a dash-separated hex notation
 28  * - ‘[mM]R‘ For a 6-byte MAC address, Reverse order (Bluetooth)
 29  * - ‘I‘ [46] for IPv4/IPv6 addresses printed in the usual way
 30  *       IPv4 uses dot-separated decimal without leading 0‘s (1.2.3.4)
 31  *       IPv6 uses colon separated network-order 16 bit hex with leading 0‘s
 32  *       [S][pfs]
 33  *       Generic IPv4/IPv6 address (struct sockaddr *) that falls back to
 34  *       [4] or [6] and is able to print port [p], flowinfo [f], scope [s]
 35  * - ‘i‘ [46] for ‘raw‘ IPv4/IPv6 addresses
 36  *       IPv6 omits the colons (01020304...0f)
 37  *       IPv4 uses dot-separated decimal with leading 0‘s (010.123.045.006)
 38  *       [S][pfs]
 39  *       Generic IPv4/IPv6 address (struct sockaddr *) that falls back to
 40  *       [4] or [6] and is able to print port [p], flowinfo [f], scope [s]
 41  * - ‘[Ii][4S][hnbl]‘ IPv4 addresses in host, network, big or little endian order
 42  * - ‘I[6S]c‘ for IPv6 addresses printed as specified by
 43  *       http://tools.ietf.org/html/rfc5952
 44  * - ‘E[achnops]‘ For an escaped buffer, where rules are defined by combination
 45  *                of the following flags (see string_escape_mem() for the
 46  *                details):
 47  *                  a - ESCAPE_ANY
 48  *                  c - ESCAPE_SPECIAL
 49  *                  h - ESCAPE_HEX
 50  *                  n - ESCAPE_NULL
 51  *                  o - ESCAPE_OCTAL
 52  *                  p - ESCAPE_NP
 53  *                  s - ESCAPE_SPACE
 54  *                By default ESCAPE_ANY_NP is used.
 55  * - ‘U‘ For a 16 byte UUID/GUID, it prints the UUID/GUID in the form
 56  *       "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
 57  *       Options for %pU are:
 58  *         b big endian lower case hex (default)
 59  *         B big endian UPPER case hex
 60  *         l little endian lower case hex
 61  *         L little endian UPPER case hex
 62  *           big endian output byte order is:
 63  *             [0][1][2][3]-[4][5]-[6][7]-[8][9]-[10][11][12][13][14][15]
 64  *           little endian output byte order is:
 65  *             [3][2][1][0]-[5][4]-[7][6]-[8][9]-[10][11][12][13][14][15]
 66  * - ‘V‘ For a struct va_format which contains a format string * and va_list *,
 67  *       call vsnprintf(->format, *->va_list).
 68  *       Implements a "recursive vsnprintf".
 69  *       Do not use this feature without some mechanism to verify the
 70  *       correctness of the format string and va_list arguments.
 71  * - ‘K‘ For a kernel pointer that should be hidden from unprivileged users
 72  * - ‘NF‘ For a netdev_features_t
 73  * - ‘h[CDN]‘ For a variable-length buffer, it prints it as a hex string with
 74  *            a certain separator (‘ ‘ by default):
 75  *              C colon
 76  *              D dash
 77  *              N no separator
 78  *            The maximum supported length is 64 bytes of the input. Consider
 79  *            to use print_hex_dump() for the larger input.
 80  * - ‘a[pd]‘ For address types [p] phys_addr_t, [d] dma_addr_t and derivatives
 81  *           (default assumed to be phys_addr_t, passed by reference)
 82  * - ‘d[234]‘ For a dentry name (optionally 2-4 last components)
 83  * - ‘D[234]‘ Same as ‘d‘ but for a struct file
 84  * - ‘g‘ For block_device name (gendisk + partition number)
 85  * - ‘C‘ For a clock, it prints the name (Common Clock Framework) or address
 86  *       (legacy clock framework) of the clock
 87  * - ‘Cn‘ For a clock, it prints the name (Common Clock Framework) or address
 88  *        (legacy clock framework) of the clock
 89  * - ‘Cr‘ For a clock, it prints the current rate of the clock
 90  * - ‘G‘ For flags to be printed as a collection of symbolic strings that would
 91  *       construct the specific value. Supported flags given by option:
 92  *       p page flags (see struct page) given as pointer to unsigned long
 93  *       g gfp flags (GFP_* and __GFP_*) given as pointer to gfp_t
 94  *       v vma flags (VM_*) given as pointer to unsigned long
 95  * - ‘O‘ For a kobject based struct. Must be one of the following:
 96  *       - ‘OF[fnpPcCF]‘  For a device tree object
 97  *                        Without any optional arguments prints the full_name
 98  *                        f device node full_name
 99  *                        n device node name
100  *                        p device node phandle
101  *                        P device node path spec (name + @unit)
102  *                        F device node flags
103  *                        c major compatible string
104  *                        C full compatible string
105  *
106  * ** Please update also Documentation/printk-formats.txt when making changes **
107  *
108  * Note: The difference between ‘S‘ and ‘F‘ is that on ia64 and ppc64
109  * function pointers are really function descriptors, which contain a
110  * pointer to the real address.
111  */
112 static noinline_for_stack
113 char *pointer(const char *fmt, char *buf, char *end, void *ptr,
114           struct printf_spec spec)
115 
116     const int default_width = 2 * sizeof(void *);
117 
118     if (!ptr && *fmt != ‘K‘) 
119         /*
120          * Print (null) with the same width as a pointer so it makes
121          * tabular output look nice.
122          */
123         if (spec.field_width == -1)
124             spec.field_width = default_width;
125         return string(buf, end, "(null)", spec);
126     
127 
128     switch (*fmt) 
129     case ‘F‘:
130     case ‘f‘:
131         ptr = dereference_function_descriptor(ptr);
132         /* Fallthrough */
133     case ‘S‘:
134     case ‘s‘:
135     case ‘B‘:
136         return symbol_string(buf, end, ptr, spec, fmt);
137     case ‘R‘:
138     case ‘r‘:
139         return resource_string(buf, end, ptr, spec, fmt);
140     case ‘h‘:
141         return hex_string(buf, end, ptr, spec, fmt);
142     case ‘b‘:
143         switch (fmt[1]) 
144         case ‘l‘:
145             return bitmap_list_string(buf, end, ptr, spec, fmt);
146         default:
147             return bitmap_string(buf, end, ptr, spec, fmt);
148         
149     case ‘M‘:            /* Colon separated: 00:01:02:03:04:05 */
150     case ‘m‘:            /* Contiguous: 000102030405 */
151                     /* [mM]F (FDDI) */
152                     /* [mM]R (Reverse order; Bluetooth) */
153         return mac_address_string(buf, end, ptr, spec, fmt);
154     case ‘I‘:            /* Formatted IP supported
155                      * 4:    1.2.3.4
156                      * 6:    0001:0203:...:0708
157                      * 6c:    1::708 or 1::1.2.3.4
158                      */
159     case ‘i‘:            /* Contiguous:
160                      * 4:    001.002.003.004
161                      * 6:   000102...0f
162                      */
163         switch (fmt[1]) 
164         case ‘6‘:
165             return ip6_addr_string(buf, end, ptr, spec, fmt);
166         case ‘4‘:
167             return ip4_addr_string(buf, end, ptr, spec, fmt);
168         case ‘S‘: 
169             const union 
170                 struct sockaddr        raw;
171                 struct sockaddr_in    v4;
172                 struct sockaddr_in6    v6;
173              *sa = ptr;
174 
175             switch (sa->raw.sa_family) 
176             case AF_INET:
177                 return ip4_addr_string_sa(buf, end, &sa->v4, spec, fmt);
178             case AF_INET6:
179                 return ip6_addr_string_sa(buf, end, &sa->v6, spec, fmt);
180             default:
181                 return string(buf, end, "(invalid address)", spec);
182             
183         
184         break;
185     case ‘E‘:
186         return escaped_string(buf, end, ptr, spec, fmt);
187     case ‘U‘:
188         return uuid_string(buf, end, ptr, spec, fmt);
189     case ‘V‘:
190         
191             va_list va;
192 
193             va_copy(va, *((struct va_format *)ptr)->va);
194             buf += vsnprintf(buf, end > buf ? end - buf : 0,
195                      ((struct va_format *)ptr)->fmt, va);
196             va_end(va);
197             return buf;
198         
199     case ‘K‘:
200         switch (kptr_restrict) 
201         case 0:
202             /* Always print %pK values */
203             break;
204         case 1: 
205             const struct cred *cred;
206 
207             /*
208              * kptr_restrict==1 cannot be used in IRQ context
209              * because its test for CAP_SYSLOG would be meaningless.
210              */
211             if (in_irq() || in_serving_softirq() || in_nmi()) 
212                 if (spec.field_width == -1)
213                     spec.field_width = default_width;
214                 return string(buf, end, "pK-error", spec);
215             
216 
217             /*
218              * Only print the real pointer value if the current
219              * process has CAP_SYSLOG and is running with the
220              * same credentials it started with. This is because
221              * access to files is checked at open() time, but %pK
222              * checks permission at read() time. We don‘t want to
223              * leak pointer values if a binary opens a file using
224              * %pK and then elevates privileges before reading it.
225              */
226             cred = current_cred();
227             if (!has_capability_noaudit(current, CAP_SYSLOG) ||
228                 !uid_eq(cred->euid, cred->uid) ||
229                 !gid_eq(cred->egid, cred->gid))
230                 ptr = NULL;
231             break;
232         
233         case 2:
234         default:
235             /* Always print 0‘s for %pK */
236             ptr = NULL;
237             break;
238         
239         break;
240 
241     case ‘N‘:
242         return netdev_bits(buf, end, ptr, fmt);
243     case ‘a‘:
244         return address_val(buf, end, ptr, fmt);
245     case ‘d‘:
246         return dentry_name(buf, end, ptr, spec, fmt);
247     case ‘C‘:
248         return clock(buf, end, ptr, spec, fmt);
249     case ‘D‘:
250         return dentry_name(buf, end,
251                    ((const struct file *)ptr)->f_path.dentry,
252                    spec, fmt);
253 #ifdef CONFIG_BLOCK
254     case ‘g‘:
255         return bdev_name(buf, end, ptr, spec, fmt);
256 #endif
257 
258     case ‘G‘:
259         return flags_string(buf, end, ptr, fmt);
260     case ‘O‘:
261         switch (fmt[1]) 
262         case ‘F‘:
263             return device_node_string(buf, end, ptr, spec, fmt + 1);
264         
265     
266     spec.flags |= SMALL;
267     if (spec.field_width == -1) 
268         spec.field_width = default_width;
269         spec.flags |= ZEROPAD;
270     
271     spec.base = 16;
272 
273     return number(buf, end, (unsigned long) ptr, spec);
274 

第136行， symbol_string(buf, end, ptr, spec, fmt);

 1 static noinline_for_stack
 2 char *symbol_string(char *buf, char *end, void *ptr,
 3             struct printf_spec spec, const char *fmt)
 4 
 5     unsigned long value;
 6 #ifdef CONFIG_KALLSYMS
 7     char sym[KSYM_SYMBOL_LEN];
 8 #endif
 9 
10     if (fmt[1] == ‘R‘)
11         ptr = __builtin_extract_return_addr(ptr);
12     value = (unsigned long)ptr;
13 
14 #ifdef CONFIG_KALLSYMS
15     if (*fmt == ‘B‘)
16         sprint_backtrace(sym, value);
17     else if (*fmt != ‘f‘ && *fmt != ‘s‘)
18         sprint_symbol(sym, value);
19     else
20         sprint_symbol_no_offset(sym, value);
21 
22     return string(buf, end, sym, spec);
23 #else
24     return special_hex_number(buf, end, value, sizeof(void *));
25 #endif
26 

第20行：sprint_symbol_no_offset

 1 /**
 2  * sprint_symbol_no_offset - Look up a kernel symbol and return it in a text buffer
 3  * @buffer: buffer to be stored
 4  * @address: address to lookup
 5  *
 6  * This function looks up a kernel symbol with @address and stores its name
 7  * and module name to @buffer if possible. If no symbol was found, just saves
 8  * its @address as is.
 9  *
10  * This function returns the number of bytes stored in @buffer.
11  */
12 int sprint_symbol_no_offset(char *buffer, unsigned long address)
13 
14     return __sprint_symbol(buffer, address, 0, 0);
15 

 

完。