On 01/02/14 06:44, Mike Frysinger wrote:
> On Wednesday 01 January 2014 05:38:20 Michael Kerrisk (man-pages) wrote:
>> On 12/31/13 20:41, Mike Frysinger wrote:
>>> +The "vDSO" is a small shared library that the kernel automatically maps
>>> into the +address space of all user-space applications.
>>> +Applications themselves usually need not concern themselves with these
>>> details +as the vDSO is most commonly called by the C library.
>>> +This way you can write using standard functions and the C library will
>>> take care
>>
>> After "write" I added "programs". Okay?
>
> you can write libraries too, but i think either wording is fine. or maybe
> change "write" to "code" ?
Okay. I made it:
This way you can code in the normal way using standard functions
>>> +Why does the vDSO exist at all?
>>> +There are some facilities the kernel provides that user space ends up
>>> using
>>
>> I changed "facilities" to "system calls". Okay?
>
> that wasn't exactly what i was going for, but the nuances are probably lost,
> so it doesn't matter (the vDSO isn't purely a replacement for syscalls).
I wondered whether you intended further nuances, but after mentioning
"facilities", you seemed to discuss only system calls.
[...]
>>> Note that the vDSO that is used is based on the ABI of your user-space
>>> code and not the ABI of the kernel.
>>> i.e. If you run an i386 32-bit ELF under an i386 32-bit kernel or under
>>> an x86_64 64-bit kernel, you'll get the same vDSO.
>>> So when referring to sections below, use the user-space ABI.
>>
>> I still can't make any sense of that last sentence. What are "sections"
>> in this context?
>
> "sections" refers to the .SS stuff following this paragraph. e.g.
> .SS i386 functions
> .SS x86_64 functions
> .SS x86/x32 functions
>
> so if your userspace program is compiled as a 32bit i386 ELF, you should refer
> to the "i386 functions" section even if your kernel is a 64bit x86_64 build.
> but if your userspace program is a 64bit x86_64 program, then refer to the
> x86_64 section. a single kernel can support many ABIs and execute them
> simultaneously. but the vDSO that is available is determined by the format of
> your program, not the kernel.
>
>> What does it mean to "*use* the user-space ABI"?
>
> use the userspace ABI as the index into the following sections.
Thanks. I reworked that to:
Thus, the name of the user-space ABI should be used to determine
which of the sections below is relevant.
>>> +.SS aarch64 functions
>>> +.\" See linux/arch/arm64/kernel/vdso/vdso.lds.S
>>> +.if t \{\
>>> +.ft CW
>>> +\}
>>> +.TS
>>> +l l.
>>> +symbol version
>>
>> You don't explicitly say what tables such as the below are about.
>> Could you provide me with a sentence to describe them?
>
> i only documented the deviations as they don't follow the vDSO standards (ELF
> object that has dynamic symbol information available). all the standard ones
> may follow Documentation/ABI/stable/vdso/ and Documentation/vDSO/*. but i
> guess a one line sentence could be added to each of these telling people to
> look at the kernel's vDSO/ dir for more details.
> -mike
I reworked here somewhat, putting the arch-specific details under a
new .SH heading, and adding
The table below lists the symbols exported by the vDSO.
at the start of many of the subsections.
I've appended the updated page. Could you please take a quick look
to make sure it's okay.
Cheers,
Michael
.\" Written by Mike Frysinger <vapier@xxxxxxxxxx>
.\"
.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" This page is in the public domain.
.\" %%%LICENSE_END
.\"
.\" Useful background:
.\" http://articles.manugarg.com/systemcallinlinux2_6.html
.\" https://lwn.net/Articles/446528/
.\" http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken
.\" http://www.trilithium.com/johan/2005/08/linux-gate/
.\"
.TH VDSO 7 2014-01-01 "Linux" "Linux Programmer's Manual"
.SH NAME
vDSO \- overview of the virtual ELF dynamic shared object
.SH SYNOPSIS
.B #include <sys/auxv.h>
.B void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
.SH DESCRIPTION
The "vDSO" is a small shared library that
the kernel automatically maps into the
address space of all user-space applications.
Applications usually do not need to concern themselves with these details
as the vDSO is most commonly called by the C library.
This way you can code in the normal way using standard functions
and the C library will take care
of using any functionality that is available via the vDSO.
Why does the vDSO exist at all?
There are some system calls the kernel provides that
user space code ends up using frequently,
to the point that such calls can dominate overall performance.
This is due both to the frequency of the call as well as the
context-switch overhead that results from
from exiting user space and entering the kernel.
The rest of this documentation is geared toward the curious and/or
C library writers rather than general developers.
If you're trying to call the vDSO in your own application rather than using
the C library, you're most likely doing it wrong.
.SS Example background
Making system calls can be slow.
In x86 32-bit systems, you can trigger a software interrupt
.RI ( "int $0x80" )
to tell the kernel you wish to make a system call.
However, this instruction is expensive: it goes through
the full interrupt-handling paths
in the processor's microcode as well as in the kernel.
Newer processors have faster (but backward incompatible) instructions to
initiate system calls.
Rather than require the C library to figure out if this functionality is
available at run time,
the C library can use functions provided by the kernel in
the vDSO.
Note that the terminology can be confusing.
On x86 systems, the vDSO function
used to determine the preferred method of making a system call is
named "__kernel_vsyscall", but on x86_64,
the term "vsyscall" also refers to an obsolete way to ask the kernel
what time it is or what CPU the caller is on.
One frequently used system call is
.BR gettimeofday (2).
This system call is called both directly by user-space applications
as well as indirectly by
the C library.
Think timestamps or timing loops or polling\(emall of these
frequently need to know what time it is right now.
This information is also not secret\(emany application in any
privilege mode (root or any unprivileged user) will get the same answer.
Thus the kernel arranges for the information required to answer
this question to be placed in memory the process can access.
Now a call to
.BR gettimeofday (2)
changes from a system call to a normal function
call and a few memory accesses.
.SS Finding the vDSO
The base address of the vDSO (if one exists) is passed by the kernel to
each program in the initial auxiliary vector (see
.BR getauxval (3)),
via the
.B AT_SYSINFO_EHDR
tag.
You must not assume the vDSO is mapped at any particular location in the
user's memory map.
The base address will usually be randomized at run time every time a new
process image is created (at
.BR execve (2)
time).
This is done for security reasons,
to prevent "return-to-libc" attacks.
For some architectures, there is also an
.B AT_SYSINFO
tag.
This is used only for locating the vsyscall entry point and is frequently
omitted or set to 0 (meaning it's not available).
This tag is a throwback to the initial vDSO work (see
.IR History
below) and its use should be avoided.
.SS File format
Since the vDSO is a fully formed ELF image, you can do symbol lookups on it.
This allows new symbols to be added with newer kernel releases,
and allows the C library to detect available functionality at
run time when running under different kernel versions.
Oftentimes the C library will do detection with the first call and then
cache the result for subsequent calls.
All symbols are also versioned (using the GNU version format).
This allows the kernel to update the function signature without breaking
backward compatibility.
This means changing the arguments that the function accepts as well as the
return value.
Thus, when looking up a symbol in the vDSO,
you must always include the version
to match the ABI you expect.
Typically the vDSO follows the naming convention of prefixing
all symbols with "__vdso_" or "__kernel_"
so as to distinguish them from other standard symbols.
For example, the "gettimeofday" function is named "__vdso_gettimeofday".
You use the standard C calling conventions when calling
any of these functions.
No need to worry about weird register or stack behavior.
.SH NOTES
.SS Source
When you compile the kernel,
it will automatically compile and link the vDSO code for you.
You will frequently find it under the architecture-specific directory:
find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
.SS vDSO names
The name of vDSO shared object varies across architectures.
It will often show up in things like glibc's
.BR ldd (1)
output.
The exact name should not matter to any code, so do not hardcode it.
.if t \{\
.ft CW
\}
.TS
l l.
user ABI vDSO name
_
aarch64 linux-vdso.so.1
ia64 linux-gate.so.1
ppc/32 linux-vdso32.so.1
ppc/64 linux-vdso64.so.1
s390 linux-vdso32.so.1
s390x linux-vdso64.so.1
sh linux-gate.so.1
i386 linux-gate.so.1
x86_64 linux-vdso.so.1
x86/x32 linux-vdso.so.1
.TE
.if t \{\
.in
.ft P
\}
.SH ARCHITECTURE_SPECIFIC NOTES
The subsections below provide architecture-specific notes
on the vDSO.
Note that the vDSO that is used is based on the ABI of your user-space code
and not the ABI of the kernel.
Thus, for example,
when you run an i386 32-bit ELF binary,
you'll get the same vDSO regardless of whether you run it under
an i386 32-bit kernel or under an x86_64 64-bit kernel.
Thus, the name of the user-space ABI should be used to determine
which of the sections below is relevant.
.SS ARM functions
.\" See linux/arch/arm/kernel/entry-armv.S
.\" See linux/Documentation/arm/kernel_user_helpers.txt
The ARM port has a code page full of utility functions.
Since it's just a raw page of code, there is no ELF information for doing
symbol lookups or versioning.
It does provide support for different versions though.
For information on this code page,
it's best to refer to the kernel documentation
as it's extremely detailed and covers everything you need to know:
.IR Documentation/arm/kernel_user_helpers.txt .
.SS aarch64 functions
.\" See linux/arch/arm64/kernel/vdso/vdso.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_rt_sigreturn LINUX_2.6.39
__kernel_gettimeofday LINUX_2.6.39
__kernel_clock_gettime LINUX_2.6.39
__kernel_clock_getres LINUX_2.6.39
.TE
.if t \{\
.in
.ft P
\}
.SS bfin (Blackfin) functions
.\" See linux/arch/blackfin/kernel/fixed_code.S
.\" See http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
As this CPU lacks a memory management unit (MMU),
it doesn't set up a vDSO in the normal sense.
Instead, it maps at boot time a few raw functions into
a fixed location in memory.
User-space applications then call directly into that region.
There is no provision for backward compatibility
beyond sniffing raw opcodes,
but as this is an embedded CPU, it can get away with things\(emsome of the
object formats it runs aren't even ELF based (they're bFLT/FLAT).
For information on this code page,
it's best to refer to the public documentation:
.br
http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
.SS ia64 (Itanium) functions
.\" See linux/arch/ia64/kernel/gate.lds.S
.\" Also linux/arch/ia64/kernel/fsys.S and linux/Documentation/ia64/fsys.txt
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_sigtramp LINUX_2.5
__kernel_syscall_via_break LINUX_2.5
__kernel_syscall_via_epc LINUX_2.5
.TE
.if t \{\
.in
.ft P
\}
The Itanium port is somewhat tricky.
In addition to the vDSO above, it also has "light-weight system calls"
(also known as "fast syscalls" or "fsys").
You can invoke these via the
.I __kernel_syscall_via_epc
vDSO helper.
The system calls listed here have the same semantics as if you called them
directly via
.BR syscall (2),
so refer to the relevant
documentation for each.
The table below lists the functions available via this mechanism.
.if t \{\
.ft CW
\}
.TS
l.
function
_
clock_gettime
getcpu
getpid
getppid
gettimeofday
set_tid_address
.TE
.if t \{\
.in
.ft P
\}
.SS parisc (hppa) functions
.\" See linux/arch/parisc/kernel/syscall.S
.\" See linux/Documentation/parisc/registers
The parisc port has a code page full of utility functions
called a gateway page.
Rather than use the normal ELF auxiliary vector approach,
it passes the address of
the page to the process via the SR2 register.
The permissions on the page are such that merely executing those addresses
automatically executes with kernel privileges and not in user-space.
This is done to match the way HP-UX works.
Since it's just a raw page of code, there is no ELF information for doing
symbol lookups or versioning.
Simply call into the appropriate offset via the branch instruction,
for example:
ble <offset>(%sr2, %r0)
.if t \{\
.ft CW
\}
.TS
l l.
offset function
_
00b0 lws_entry
00e0 set_thread_pointer
0100 linux_gateway_entry (syscall)
0268 syscall_nosys
0274 tracesys
0324 tracesys_next
0368 tracesys_exit
03a0 tracesys_sigexit
03b8 lws_start
03dc lws_exit_nosys
03e0 lws_exit
03e4 lws_compare_and_swap64
03e8 lws_compare_and_swap
0404 cas_wouldblock
0410 cas_action
.TE
.if t \{\
.in
.ft P
\}
.SS ppc/32 functions
.\" See linux/arch/powerpc/kernel/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
The functions marked with a
.I *
are available only when the kernel is
a PowerPC64 (64-bit) kernel.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_clock_getres LINUX_2.6.15
__kernel_clock_gettime LINUX_2.6.15
__kernel_datapage_offset LINUX_2.6.15
__kernel_get_syscall_map LINUX_2.6.15
__kernel_get_tbfreq LINUX_2.6.15
__kernel_getcpu \fI*\fR LINUX_2.6.15
__kernel_gettimeofday LINUX_2.6.15
__kernel_sigtramp_rt32 LINUX_2.6.15
__kernel_sigtramp32 LINUX_2.6.15
__kernel_sync_dicache LINUX_2.6.15
__kernel_sync_dicache_p5 LINUX_2.6.15
.TE
.if t \{\
.in
.ft P
\}
.SS ppc/64 functions
.\" See linux/arch/powerpc/kernel/vdso64/vdso64.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_clock_getres LINUX_2.6.15
__kernel_clock_gettime LINUX_2.6.15
__kernel_datapage_offset LINUX_2.6.15
__kernel_get_syscall_map LINUX_2.6.15
__kernel_get_tbfreq LINUX_2.6.15
__kernel_getcpu LINUX_2.6.15
__kernel_gettimeofday LINUX_2.6.15
__kernel_sigtramp_rt64 LINUX_2.6.15
__kernel_sync_dicache LINUX_2.6.15
__kernel_sync_dicache_p5 LINUX_2.6.15
.TE
.if t \{\
.in
.ft P
\}
.SS s390 functions
.\" See linux/arch/s390/kernel/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_clock_getres LINUX_2.6.29
__kernel_clock_gettime LINUX_2.6.29
__kernel_gettimeofday LINUX_2.6.29
.TE
.if t \{\
.in
.ft P
\}
.SS s390x functions
.\" See linux/arch/s390/kernel/vdso64/vdso64.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_clock_getres LINUX_2.6.29
__kernel_clock_gettime LINUX_2.6.29
__kernel_gettimeofday LINUX_2.6.29
.TE
.if t \{\
.in
.ft P
\}
.SS sh (SuperH) functions
.\" See linux/arch/sh/kernel/vsyscall/vsyscall.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_rt_sigreturn LINUX_2.6
__kernel_sigreturn LINUX_2.6
__kernel_vsyscall LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS i386 functions
.\" See linux/arch/x86/vdso/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__kernel_sigreturn LINUX_2.5
__kernel_rt_sigreturn LINUX_2.5
__kernel_vsyscall LINUX_2.5
.TE
.if t \{\
.in
.ft P
\}
.SS x86_64 functions
.\" See linux/arch/x86/vdso/vdso.lds.S
The table below lists the symbols exported by the vDSO.
All of these symbols are also available without the "__vdso_" prefix, but
you should ignore those and stick to the names below.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__vdso_clock_gettime LINUX_2.6
__vdso_getcpu LINUX_2.6
__vdso_gettimeofday LINUX_2.6
__vdso_time LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS x86/x32 functions
.\" See linux/arch/x86/vdso/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol version
_
__vdso_clock_gettime LINUX_2.6
__vdso_getcpu LINUX_2.6
__vdso_gettimeofday LINUX_2.6
__vdso_time LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS History
The vDSO was originally just a single function\(emthe vsyscall.
In older kernels, you might see that name
in a process's memory map rather than "vdso".
Over time, people realized that this mechanism
was a great way to pass more functionality
to user space, so it was reconceived as a vDSO in the current format.
.SH SEE ALSO
.BR syscalls (2),
.BR getauxval (3),
.BR proc (5)
The documents, examples, and source code in the Linux source code tree:
.in +4n
.nf
Documentation/ABI/stable/vdso
Documentation/ia64/fsys.txt
Documentation/vDSO/* (includes examples of using the vDSO)
find arch/ -iname '*vdso*' -o -iname '*gate*'
.fi
.in
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