---
man2/syscall.2 | 6 +-
man2/syscalls.2 | 3 +-
man3/getauxval.3 | 4 +-
man7/libc.7 | 5 +-
man7/vdso.7 | 457 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 468 insertions(+), 7 deletions(-)
create mode 100644 man7/vdso.7
diff --git a/man2/syscall.2 b/man2/syscall.2
index e712b41..fe5f86d 100644
--- a/man2/syscall.2
+++ b/man2/syscall.2
@@ -145,7 +145,8 @@ The details for various architectures are listed in the two tables below.
The first table lists the instruction used to transition to kernel mode,
(which might not be the fastest or best way to transition to the kernel,
-so you might have to refer to the VDSO),
+so you might have to refer to
+.BR vdso (7)),
the register used to indicate the system call number,
and the register used to return the system call result.
.if t \{\
@@ -219,4 +220,5 @@ main(int argc, char *argv[])
.SH SEE ALSO
.BR _syscall (2),
.BR intro (2),
-.BR syscalls (2)
+.BR syscalls (2),
+.BR vdso (7)
diff --git a/man2/syscalls.2 b/man2/syscalls.2
index 265c654..0d085e1 100644
--- a/man2/syscalls.2
+++ b/man2/syscalls.2
@@ -833,4 +833,5 @@ and similarly
.SH SEE ALSO
.BR syscall (2),
.BR unimplemented (2),
-.BR libc (7)
+.BR libc (7),
+.BR vdso (7)
diff --git a/man3/getauxval.3 b/man3/getauxval.3
index 8f27932..09d5bdc 100755
--- a/man3/getauxval.3
+++ b/man3/getauxval.3
@@ -210,7 +210,5 @@ see
for more information.
.SH SEE ALSO
.BR secure_getenv (3),
+.BR vdso (7),
.BR ld-linux.so (8)
-
-The kernel source file
-.IR Documentation/ABI/stable/vdso
diff --git a/man7/libc.7 b/man7/libc.7
index a9aeba2..f687ced 100644
--- a/man7/libc.7
+++ b/man7/libc.7
@@ -98,6 +98,9 @@ Details of these libraries are generally not covered by the
project.
.SH SEE ALSO
.BR syscalls (2),
+.BR getauxval (3),
+.BR proc (5),
.BR feature_test_macros (7),
.BR man-pages (7),
-.BR standards (7)
+.BR standards (7),
+.BR vdso (7)
diff --git a/man7/vdso.7 b/man7/vdso.7
new file mode 100644
index 0000000..3c4b7fb
--- /dev/null
+++ b/man7/vdso.7
@@ -0,0 +1,457 @@
+.\" Written by Mike Frysinger <vapier@xxxxxxxxxx>
+.\"
+.\" %%%LICENSE_START(PUBLIC_DOMAIN)
+.\" This page is in the public domain.
+.\" %%%LICENSE_END
+.\"
+.TH VDSO 7 2013-04-09 "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 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
+of using any available functionality.
+
+Why does the vDSO exist at all?
+There are some facilities the kernel provides that user space 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 overhead
+from exiting user space and entering the kernel.
+
+The rest of this documentation is geared towards 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 (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 backwards incompatible) instructions to
+initiate system calls.
+Rather than require the C library to figure out if this functionality is
+available at runtime itself, it can use functions provided by the kernel in
+the vDSO.
+
+Note that the terminology can be confusing.
+On x86 systems, the vDSO function 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 system call frequently called is gettimeofday().
+This is called both directly by user-space applications as well as indirectly by
+the C library.
+Think timestamps or timing loops or polling -- all of these frequently need to
+know what time it is right now.
+This information is also not secret -- any application in any privilege mode
+(root or any 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 gettimeofday() 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.
+Specifically, 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 runtime every time a new
+process image is created (at
+.BR execve (2)
+time).
+This is done for security reasons to prevent standard "return-to-libc" attacks.
+
+For some architectures, there is also a
+.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).
+It is a throwback to the initial vDSO work (see
+.IR HISTORY
+below) and should be avoided.
+
+Refer to
+.BR getauxval (3)
+for more details on accessing these fields.
+.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 for the
+C library to detect available functionality at runtime when running under
+different kernel versions.
+Often times 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
+backwards 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.
+e.g. 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 dir:
+
+ find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
+
+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.
+.SS vDSO names
+The name of this shared object varies across architectures.
+It will often show up in things like glibc's `ldd` 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
+\}
+.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 documentation on this code page, it's better you refer to the kernel doc
+as it's extremely detailed and covers everything you need to know:
+.br
+Documentation/arm/kernel_user_helpers.txt
+.SS aarch64 functions
+.\" See linux/arch/arm64/kernel/vdso/vdso.lds.S
+.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 backwards compatibility beyond sniffing raw opcodes,
+but as this is an embedded CPU, it can get away with things -- some of the
+object formats it runs aren't even ELF based (they're bFLT/FLAT).
+
+For documentation on this code page, it's better you refer to the public docs:
+.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
+.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 actually likes to get 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 __kernel_syscall_via_epc vDSO helper.
+The system calls listed here have the same semantics as if you called them
+directly via
+.BR syscall (3),
+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 aux 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, e.g.:
+.br
+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 functions marked with a
+.I *
+below are only available 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
+.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
+.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
+.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
+.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
+.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
+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
+.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 -- the vsyscall.
+In older kernels, you might see that in a process's memory map rather than vdso.
+Over time, people realized that this 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 docs/examples/sources in the Linux sources:
+.nf
+Documentation/ABI/stable/vdso
+linux/Documentation/ia64/fsys.txt
+Documentation/vDSO/* (includes examples of using the vDSO)
+find arch/ -iname '*vdso*' -o -iname '*gate*'
+.fi
--
1.8.4.3
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