Hi Mike,
On 04/12/13 03:28, Mike Frysinger wrote:
> here's v2 w/Andy's feedback
Thanks for this--it's a nice piece of work. Could you take a
look at my comments below and send a v3, please.
> .\" Written by Mike Frysinger <vapier@xxxxxxxxxx>
> .\"
> .\" %%%LICENSE_START(PUBLIC_DOMAIN)
> .\" This page is in the public domain. Suck it.
Okay -- not my first choice for a license, but so be it.
But, how about we lose the "Suck it."...
> .\" %%%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);
Add space before "getauxval". (Usual convention for casts in code examples
in man pages.)
> .SH DESCRIPTION
> The "vDSO" is a small shared library that the kernel automatically maps into the
> address space of all userspace applications.
1,$s/userspace applications/user-space applications/
> Applications themselves usually need not concern themselves with this as it is
> most commonly called by the C library.
This last sentence doesn't quite make sense, since "this" and "it" refer to
different things (I believe). Do you want something like:
Applications generally do not need to care about the details since
the vDSO is automatically employed by the C library
?
> This way you can write using standard functions and the C library will take care
> of using any available functionality.
>
> Why does this object exist at all?
s/this object/the vDSO/
> There are some facilities the kernel provides that userspace ends up using
s/userspace/user space/
(When used as a noun, and in other places in the page as well)
> frequently to the point that such calls can dominate overall performance.
> This is due both to the frequency of the call as well as the context overhead
> from exiting userspace and entering the kernel.
>
> The rest of this documentation is geared towards the curious and/or C library
> writers rather than general developers.
> If you're trying to call the vDSO in your own application rather than using
> the C library, you're most likely doing it wrong.
> .SS Example Background
Convention for SS headings is that only the first word is capitalized (unless
English usage dictates otherwise--e.g., for a proper noun)
> Making syscalls can be slow.
1,$s/syscall/system call/
(and other instances in the page)
> In x86 32bit systems, you can trigger a software interrupt (int $0x80) to tell
s/32bit/32-bit/
> the kernel you wish to make a syscall.
> 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.
That last point (after the comma) is the most interesting (IMO) of the use
cases of the vDSO. If you cared to expand on the details (i.e., are what
are mechanics of the operation of those functions provided by the kernel),
I think that would be interesting for the reader.
> 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.
s/cpu/CPU/
> Another frequent system call is gettimeofday().
> This is called both directly by userspace 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 syscall to a normal function call
> and a few memory accesses.
> .SS Finding The vDSO
s/The/the/
> 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 is
Missing word after "new".
> processed (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 throw back to the initial vDSO work (see
s/throw back/throwback/
> .IR HISTORY
> below) and should be avoided.
>
> Refer to
> .BR getauxval (3)
> for more details on accessing these fields.
> .SS File Format
s/Format/format/
> Since the vDSO is a fully formed ELF, you can do symbol lookups on it.
Missing word after ELF.
> 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 (in the very unlikely situation) to update the function
s/situation/case that it is necessary/
> signature without breaking backwards compatibility.
> This means changing the arguments that it accepts as well as the return value.
What is "it" in the previous line? (Please replace with a suitable noun.)
> When looking up a symbol in the vDSO, you must always include the version you
> are writing against.
>
> Typically the vDSO follows the naming convention of prefixing all symbols with
> "__vdso_" or "__kernel_" so as to distinguish from other standard symbols.
s/distinguish/distinguish them/
> 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.
That last sentence is a little incomplete. Could you expand/reword a little
please.
> .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 arch specific dir:
s/arch specific dir/architecture-specific directory/
> .br
Change that last to a blank line, and then indent the next line by 4 spaces.
> find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
>
> Note that the vDSO that is used is based on the ABI of your userspace code
> and not the ABI of the kernel.
> i.e. If you run an i386 32bit ELF under an i386 32bit kernel or under an
s/i.e. If/In other words, if/
s/32bit/32-big/g
> x86_64 64bit kernel, you'll get the same vDSO.
s/64bit/64-bit/
> So when referring to sections below, use the userspace ABI.
It's not clear what you mean here when you say "use the userspace ABI."
Could you clarify?
> .SS vDSO Names
s/Names/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 please do not hardcode it.
s/please//
> .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 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
Thanks -- adding references like the above in the source is helpful
for future maintenance.
> As this cpu lacks a MMU, it doesn't setup a vDSO in the normal sense.
s/cpu/CPU/
s/MMU/memory-management unit (MMU)/
s/setup/set up/
> Instead, it maps at boot time a few raw functions into a fixed location in
> memory.
> Userspace apps then call directly into that.
s/apps/applications/
> 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 format, 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" aka
s/aka/also known as/
> "fast syscalls" aka "fsys".
s/aka/or/
> 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 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 (64bit) kernel.
s/64bit/64-bit/
> .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
> Each of these symbols are also available without the "__vdso_" prefix, but
Either:
s/Each of these symbols are/All of these symbols are/
or
s/Each of these symbols are/Each of these symbols is/
> 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
> \}
> .SH HISTORY
Better to have this as
.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.
> Overtime, people realized that this was a great way to pass more functionality
s/Overtime/Over time/
> to userspace, 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
>
In the next iteration, could you include a second (separate) patch to
syscalls.2 and getauxval.3 that adds
.BR vdso (7)
under SEE ALSO.
Thanks,
Michael
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