How to get printk format specifiers right¶
- Author:
Randy Dunlap <rdunlap@infradead.org>
- Author:
Andrew Murray <amurray@mpc-data.co.uk>
Integer types¶
If variable is of Type, use printk format specifier:
------------------------------------------------------------
char %d or %x
unsigned char %u or %x
short int %d or %x
unsigned short int %u or %x
int %d or %x
unsigned int %u or %x
long %ld or %lx
unsigned long %lu or %lx
long long %lld or %llx
unsigned long long %llu or %llx
size_t %zu or %zx
ssize_t %zd or %zx
s8 %d or %x
u8 %u or %x
s16 %d or %x
u16 %u or %x
s32 %d or %x
u32 %u or %x
s64 %lld or %llx
u64 %llu or %llx
If <type> is architecture-dependent for its size (e.g., cycles_t, tcflag_t) or is dependent on a config option for its size (e.g., blk_status_t), use a format specifier of its largest possible type and explicitly cast to it.
Example:
printk("test: latency: %llu cycles\n", (unsigned long long)time);
Reminder: sizeof() returns type size_t.
The kernel’s printf does not support %n. Floating point formats (%e, %f,
%g, %a) are also not recognized, for obvious reasons. Use of any
unsupported specifier or length qualifier results in a WARN and early
return from vsnprintf().
Pointer types¶
A raw pointer value may be printed with %p which will hash the address before printing. The kernel also supports extended specifiers for printing pointers of different types.
Some of the extended specifiers print the data on the given address instead of printing the address itself. In this case, the following error messages might be printed instead of the unreachable information:
(null) data on plain NULL address
(efault) data on invalid address
(einval) invalid data on a valid address
Plain Pointers¶
%p abcdef12 or 00000000abcdef12
Pointers printed without a specifier extension (i.e unadorned %p) are
hashed to prevent leaking information about the kernel memory layout. This
has the added benefit of providing a unique identifier. On 64-bit machines
the first 32 bits are zeroed. The kernel will print (ptrval) until it
gathers enough entropy.
When possible, use specialised modifiers such as %pS or %pB (described below)
to avoid the need of providing an unhashed address that has to be interpreted
post-hoc. If not possible, and the aim of printing the address is to provide
more information for debugging, use %p and boot the kernel with the
no_hash_pointers parameter during debugging, which will print all %p
addresses unmodified. If you really always want the unmodified address, see
%px below.
If (and only if) you are printing addresses as a content of a virtual file in
e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a
userspace process, use the %pK modifier described below instead of %p or %px.
Error Pointers¶
%pe -ENOSPC
For printing error pointers (i.e. a pointer for which IS_ERR() is true) as a symbolic error name. Error values for which no symbolic name is known are printed in decimal, while a non-ERR_PTR passed as the argument to %pe gets treated as ordinary %p.
Symbols/Function Pointers¶
%pS versatile_init+0x0/0x110
%ps versatile_init
%pSR versatile_init+0x9/0x110
(with __builtin_extract_return_addr() translation)
%pB prev_fn_of_versatile_init+0x88/0x88
The S and s specifiers are used for printing a pointer in symbolic
format. They result in the symbol name with (S) or without (s)
offsets. If KALLSYMS are disabled then the symbol address is printed instead.
The B specifier results in the symbol name with offsets and should be
used when printing stack backtraces. The specifier takes into
consideration the effect of compiler optimisations which may occur
when tail-calls are used and marked with the noreturn GCC attribute.
If the pointer is within a module, the module name and optionally build ID is
printed after the symbol name with an extra b appended to the end of the
specifier.
%pS versatile_init+0x0/0x110 [module_name]
%pSb versatile_init+0x0/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
%pSRb versatile_init+0x9/0x110 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
(with __builtin_extract_return_addr() translation)
%pBb prev_fn_of_versatile_init+0x88/0x88 [module_name ed5019fdf5e53be37cb1ba7899292d7e143b259e]
Probed Pointers from BPF / tracing¶
%pks kernel string
%pus user string
The k and u specifiers are used for printing prior probed memory from
either kernel memory (k) or user memory (u). The subsequent s specifier
results in printing a string. For direct use in regular vsnprintf() the (k)
and (u) annotation is ignored, however, when used out of BPF’s bpf_trace_printk(),
for example, it reads the memory it is pointing to without faulting.
Kernel Pointers¶
%pK 01234567 or 0123456789abcdef
For printing kernel pointers which should be hidden from unprivileged users. The behaviour of %pK depends on the kptr_restrict sysctl - see Documentation for /proc/sys/kernel/ for more details.
This modifier is only intended when producing content of a file read by
userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the
section about %p above for discussion about how to manage hashing pointers
in printk().
Unmodified Addresses¶
%px 01234567 or 0123456789abcdef
For printing pointers when you really want to print the address. Please consider whether or not you are leaking sensitive information about the kernel memory layout before printing pointers with %px. %px is functionally equivalent to %lx (or %lu). %px is preferred because it is more uniquely grep’able. If in the future we need to modify the way the kernel handles printing pointers we will be better equipped to find the call sites.
Before using %px, consider if using %p is sufficient together with enabling the
no_hash_pointers kernel parameter during debugging sessions (see the %p
description above). One valid scenario for %px might be printing information
immediately before a panic, which prevents any sensitive information to be
exploited anyway, and with %px there would be no need to reproduce the panic
with no_hash_pointers.
Pointer Differences¶
%td 2560
%tx a00
For printing the pointer differences, use the %t modifier for ptrdiff_t.
Example:
printk("test: difference between pointers: %td\n", ptr2 - ptr1);
Struct Resources¶
%pr [mem 0x60000000-0x6fffffff flags 0x2200] or
[mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
%pR [mem 0x60000000-0x6fffffff pref] or
[mem 0x0000000060000000-0x000000006fffffff pref]
For printing struct resources. The R and r specifiers result in a
printed resource with (R) or without (r) a decoded flags member.
Passed by reference.
Physical address types phys_addr_t¶
%pa[p] 0x01234567 or 0x0123456789abcdef
For printing a phys_addr_t type (and its derivatives, such as resource_size_t) which can vary based on build options, regardless of the width of the CPU data path.
Passed by reference.
DMA address types dma_addr_t¶
%pad 0x01234567 or 0x0123456789abcdef
For printing a dma_addr_t type which can vary based on build options, regardless of the width of the CPU data path.
Passed by reference.
Raw buffer as an escaped string¶
%*pE[achnops]
For printing raw buffer as an escaped string. For the following buffer:
1b 62 20 5c 43 07 22 90 0d 5d
A few examples show how the conversion would be done (excluding surrounding quotes):
%*pE "\eb \C\a"\220\r]"
%*pEhp "\x1bb \C\x07"\x90\x0d]"
%*pEa "\e\142\040\\\103\a\042\220\r\135"
The conversion rules are applied according to an optional combination
of flags (see string_escape_mem() kernel documentation for the
details):
a - ESCAPE_ANY
c - ESCAPE_SPECIAL
h - ESCAPE_HEX
n - ESCAPE_NULL
o - ESCAPE_OCTAL
p - ESCAPE_NP
s - ESCAPE_SPACE
By default ESCAPE_ANY_NP is used.
ESCAPE_ANY_NP is the sane choice for many cases, in particularly for printing SSIDs.
If field width is omitted then 1 byte only will be escaped.
Raw buffer as a hex string¶
%*ph 00 01 02 ... 3f
%*phC 00:01:02: ... :3f
%*phD 00-01-02- ... -3f
%*phN 000102 ... 3f
For printing small buffers (up to 64 bytes long) as a hex string with a
certain separator. For larger buffers consider using
print_hex_dump().
MAC/FDDI addresses¶
%pM 00:01:02:03:04:05
%pMR 05:04:03:02:01:00
%pMF 00-01-02-03-04-05
%pm 000102030405
%pmR 050403020100
For printing 6-byte MAC/FDDI addresses in hex notation. The M and m
specifiers result in a printed address with (M) or without (m) byte
separators. The default byte separator is the colon (:).
Where FDDI addresses are concerned the F specifier can be used after
the M specifier to use dash (-) separators instead of the default
separator.
For Bluetooth addresses the R specifier shall be used after the M
specifier to use reversed byte order suitable for visual interpretation
of Bluetooth addresses which are in the little endian order.
Passed by reference.
IPv4 addresses¶
%pI4 1.2.3.4
%pi4 001.002.003.004
%p[Ii]4[hnbl]
For printing IPv4 dot-separated decimal addresses. The I4 and i4
specifiers result in a printed address with (i4) or without (I4) leading
zeros.
The additional h, n, b, and l specifiers are used to specify
host, network, big or little endian order addresses respectively. Where
no specifier is provided the default network/big endian order is used.
Passed by reference.
IPv6 addresses¶
%pI6 0001:0002:0003:0004:0005:0006:0007:0008
%pi6 00010002000300040005000600070008
%pI6c 1:2:3:4:5:6:7:8
For printing IPv6 network-order 16-bit hex addresses. The I6 and i6
specifiers result in a printed address with (I6) or without (i6)
colon-separators. Leading zeros are always used.
The additional c specifier can be used with the I specifier to
print a compressed IPv6 address as described by
https://tools.ietf.org/html/rfc5952
Passed by reference.
IPv4/IPv6 addresses (generic, with port, flowinfo, scope)¶
%pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
%piS 001.002.003.004 or 00010002000300040005000600070008
%pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
%pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
%p[Ii]S[pfschnbl]
For printing an IP address without the need to distinguish whether it’s of
type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
specified through IS or iS, can be passed to this format specifier.
The additional p, f, and s specifiers are used to specify port
(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a : prefix,
flowinfo a / and scope a %, each followed by the actual value.
In case of an IPv6 address the compressed IPv6 address as described by
https://tools.ietf.org/html/rfc5952 is being used if the additional
specifier c is given. The IPv6 address is surrounded by [, ] in
case of additional specifiers p, f or s as suggested by
https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
In case of IPv4 addresses, the additional h, n, b, and l
specifiers can be used as well and are ignored in case of an IPv6
address.
Passed by reference.
Further examples:
%pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
%pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
%pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
UUID/GUID addresses¶
%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
%pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
%pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
%pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
For printing 16-byte UUID/GUIDs addresses. The additional l, L,
b and B specifiers are used to specify a little endian order in
lower (l) or upper case (L) hex notation - and big endian order in lower (b)
or upper case (B) hex notation.
Where no additional specifiers are used the default big endian order with lower case hex notation will be printed.
Passed by reference.
dentry names¶
%pd{,2,3,4}
%pD{,2,3,4}
For printing dentry name; if we race with d_move(), the name might
be a mix of old and new ones, but it won’t oops. %pd dentry is a safer
equivalent of %s dentry->d_name.name we used to use, %pd<n> prints n
last components. %pD does the same thing for struct file.
Passed by reference.
block_device names¶
%pg sda, sda1 or loop0p1
For printing name of block_device pointers.
struct va_format¶
%pV
For printing struct va_format structures. These contain a format string and va_list as follows:
struct va_format {
const char *fmt;
va_list *va;
};
Implements a “recursive vsnprintf”.
Do not use this feature without some mechanism to verify the correctness of the format string and va_list arguments.
Passed by reference.
Device tree nodes¶
%pOF[fnpPcCF]
For printing device tree node structures. Default behaviour is equivalent to %pOFf.
f - device node full_name
n - device node name
p - device node phandle
P - device node path spec (name + @unit)
F - device node flags
c - major compatible string
C - full compatible string
The separator when using multiple arguments is ‘:’
Examples:
%pOF /foo/bar@0 - Node full name
%pOFf /foo/bar@0 - Same as above
%pOFfp /foo/bar@0:10 - Node full name + phandle
%pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
major compatible string +
node flags
D - dynamic
d - detached
P - Populated
B - Populated bus
Passed by reference.
Fwnode handles¶
%pfw[fP]
For printing information on fwnode handles. The default is to print the full node name, including the path. The modifiers are functionally equivalent to %pOF above.
f - full name of the node, including the path
P - the name of the node including an address (if there is one)
Examples (ACPI):
%pfwf \_SB.PCI0.CIO2.port@1.endpoint@0 - Full node name
%pfwP endpoint@0 - Node name
Examples (OF):
%pfwf /ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name
%pfwP endpoint - Node name
Time and date¶
%pt[RT] YYYY-mm-ddTHH:MM:SS
%pt[RT]s YYYY-mm-dd HH:MM:SS
%pt[RT]d YYYY-mm-dd
%pt[RT]t HH:MM:SS
%pt[RT][dt][r][s]
For printing date and time as represented by:
R struct rtc_time structure
T time64_t type
in human readable format.
By default year will be incremented by 1900 and month by 1. Use %pt[RT]r (raw) to suppress this behaviour.
The %pt[RT]s (space) will override ISO 8601 separator by using ‘ ‘ (space) instead of ‘T’ (Capital T) between date and time. It won’t have any effect when date or time is omitted.
Passed by reference.
struct clk¶
%pC pll1
%pCn pll1
For printing struct clk structures. %pC and %pCn print the name of the clock (Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
Passed by reference.
bitmap and its derivatives such as cpumask and nodemask¶
%*pb 0779
%*pbl 0,3-6,8-10
For printing bitmap and its derivatives such as cpumask and nodemask, %*pb outputs the bitmap with field width as the number of bits and %*pbl output the bitmap as range list with field width as the number of bits.
The field width is passed by value, the bitmap is passed by reference. Helper macros cpumask_pr_args() and nodemask_pr_args() are available to ease printing cpumask and nodemask.
Flags bitfields such as page flags, gfp_flags¶
%pGp 0x17ffffc0002036(referenced|uptodate|lru|active|private|node=0|zone=2|lastcpupid=0x1fffff)
%pGg GFP_USER|GFP_DMA32|GFP_NOWARN
%pGv read|exec|mayread|maywrite|mayexec|denywrite
For printing flags bitfields as a collection of symbolic constants that
would construct the value. The type of flags is given by the third
character. Currently supported are [p]age flags, [v]ma_flags (both
expect unsigned long *) and [g]fp_flags (expects gfp_t *). The flag
names and print order depends on the particular type.
Note that this format should not be used directly in the
TP_printk() part of a tracepoint. Instead, use the show_*_flags()
functions from <trace/events/mmflags.h>.
Passed by reference.
Network device features¶
%pNF 0x000000000000c000
For printing netdev_features_t.
Passed by reference.
V4L2 and DRM FourCC code (pixel format)¶
%p4cc
Print a FourCC code used by V4L2 or DRM, including format endianness and its numerical value as hexadecimal.
Passed by reference.
Examples:
%p4cc BG12 little-endian (0x32314742)
%p4cc Y10 little-endian (0x20303159)
%p4cc NV12 big-endian (0xb231564e)
Rust¶
%pA
Only intended to be used from Rust code to format core::fmt::Arguments.
Do not use it from C.
Thanks¶
If you add other %p extensions, please extend <lib/test_printf.c> with one or more test cases, if at all feasible.
Thank you for your cooperation and attention.