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diff --git a/winsup/doc/highlights.xml b/winsup/doc/highlights.xml deleted file mode 100644 index 5de789a8c..000000000 --- a/winsup/doc/highlights.xml +++ /dev/null @@ -1,384 +0,0 @@ -<?xml version="1.0" encoding='UTF-8'?> -<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook V4.5//EN" - "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"> - -<sect1 id="highlights"><title>Highlights of Cygwin Functionality</title> - -<sect2 id="ov-hi-intro"><title>Introduction</title> <para>When a binary linked -against the library is executed, the Cygwin DLL is loaded into the -application's text segment. Because we are trying to emulate a UNIX kernel -which needs access to all processes running under it, the first Cygwin DLL to -run creates shared memory areas and global synchronization objects that other -processes using separate instances of the DLL can access. This is used to keep track of open file descriptors and to assist fork and exec, among other -purposes. Every process also has a per_process structure that contains -information such as process id, user id, signal masks, and other similar -process-specific information.</para> - -<para>The DLL is implemented as a standard DLL in the Win32 subsystem. Under -the hood it's using the Win32 API, as well as the native NT API, where -appropriate.</para> - -<note><para>Some restrictions apply for calls to the Win32 API. -For details, see <xref linkend="setup-env-win32"></xref>, -as well as <xref linkend="pathnames-win32-api"></xref>.</para></note> - -<para>The native NT API is used mainly for speed, as well as to access -NT capabilities which are useful to implement certain POSIX features, but -are hidden to the Win32 API. -</para> - -<para>Due to some restrictions in Windows, it's not always possible -to strictly adhere to existing UNIX standards like POSIX.1. Fortunately -these are mostly corner cases.</para> - -<para>Note that many of the things that Cygwin does to provide POSIX -compatibility do not mesh well with the native Windows API. If you mix -POSIX calls with Windows calls in your program it is possible that you -will see uneven results. In particular, Cygwin signals will not work -with Windows functions which block and Windows functions which accept -filenames may be confused by Cygwin's support for long filenames.</para> - -</sect2> - -<sect2 id="ov-hi-perm"><title>Permissions and Security</title> -<para>Windows NT includes a sophisticated security model based on Access -Control Lists (ACLs). Cygwin maps Win32 file ownership and permissions to -ACLs by default, on file systems supporting them (usually NTFS). Solaris -style ACLs and accompanying function calls are also supported. -The chmod call maps UNIX-style permissions back to the Win32 equivalents. -Because many programs expect to be able to find the -<filename>/etc/passwd</filename> and -<filename>/etc/group</filename> files, we provide <ulink -url="http://cygwin.com/cygwin-ug-net/using-utils.html">utilities</ulink> -that can be used to construct them from the user and group information -provided by the operating system.</para> - -<para>Users with Administrator rights are permitted to chown files. -With version 1.1.3 Cygwin introduced a mechanism for setting real and -effective UIDs. This is described in <xref linkend="ntsec"></xref>. As -of version 1.5.13, the Cygwin developers are not aware of any feature in -the Cygwin DLL that would allow users to gain privileges or to access -objects to which they have no rights under Windows. However there is no -guarantee that Cygwin is as secure as the Windows it runs on. Cygwin -processes share some variables and are thus easier targets of denial of -service type of attacks. -</para> - -</sect2> - -<sect2 id="ov-hi-files"><title>File Access</title> <para>Cygwin supports -both POSIX- and Win32-style paths, using either forward or back slashes as the -directory delimiter. Paths coming into the DLL are translated from POSIX to -native NT as needed. From the application perspective, the file system is -a POSIX-compliant one. The implementation details are safely hidden in the -Cygwin DLL. UNC pathnames (starting with two slashes) are supported for -network paths.</para> - -<para>Since version 1.7.0, the layout of this POSIX view of the Windows file -system space is stored in the <filename>/etc/fstab</filename> file. Actually, -there is a system-wide <filename>/etc/fstab</filename> file as well as a -user-specific fstab file <filename>/etc/fstab.d/${USER}</filename>.</para> - -<para>At startup the DLL has to find out where it can find the -<filename>/etc/fstab</filename> file. The mechanism used for this is simple. -First it retrieves it's own path, for instance -<filename>C:\Cygwin\bin\cygwin1.dll</filename>. From there it deduces -that the root path is <filename>C:\Cygwin</filename>. So it looks for the -<filename>fstab</filename> file in <filename>C:\Cygwin\etc\fstab</filename>. -The layout of this file is very similar to the layout of the -<filename>fstab</filename> file on Linux. Just instead of block devices, -the mount points point to Win32 paths. An installation with -<command>setup.exe</command> installs a <filename>fstab</filename> file by -default, which can easily be changed using the editor of your choice.</para> - -<para>The <filename>fstab</filename> file allows mounting arbitrary Win32 -paths into the POSIX file system space. A special case is the so-called -cygdrive prefix. -It's the path under which every available drive in the system is mounted -under its drive letter. The default value is <filename>/cygdrive</filename>, -so you can access the drives as <filename>/cygdrive/c</filename>, -<filename>/cygdrive/d</filename>, etc... The cygdrive prefix can be set to -some other value (<filename>/mnt</filename> for instance) in the -<filename>fstab</filename> file(s).</para> - -<para>The library exports several Cygwin-specific functions that can be used -by external programs to convert a path or path list from Win32 to POSIX or vice -versa. Shell scripts and Makefiles cannot call these functions directly. -Instead, they can do the same path translations by executing the -<command>cygpath</command> utility program that we provide with Cygwin.</para> - -<para>Win32 applications handle filenames in a case preserving, but case -insensitive manner. Cygwin supports case sensitivity on file systems -supporting that. Since Windows XP, the OS only supports case -sensitivity when a specific registry value is changed. Therefore, case -sensitivity is not usually the default.</para> - -<para>Cygwin supports creating and reading symbolic links, even on Windows -filesystems and OS versions which don't support them. -See <xref linkend="pathnames-symlinks"></xref> for details.</para> - -<para>Hard links are fully supported on NTFS and NFS file systems. On FAT -and other file systems which don't support hardlinks, the call returns with -an error, just like on other POSIX systems.</para> - -<para>On file systems which don't support unique persistent file IDs (FAT, -older Samba shares) the inode number for a file is calculated by hashing its -full Win32 path. The inode number generated by the stat call always matches -the one returned in <literal>d_ino</literal> of the <literal>dirent</literal> -structure. It is worth noting that the number produced by this method is not -guaranteed to be unique. However, we have not found this to be a significant -problem because of the low probability of generating a duplicate inode number. -</para> - -<para>Cygwin 1.7 and later supports Extended Attributes (EAs) via the -linux-specific function calls <function>getxattr</function>, -<function>setxattr</function>, <function>listxattr</function>, and -<function>removexattr</function>. All EAs on Samba or NTFS are treated as -user EAs, so, if the name of an EA is "foo" from the Windows perspective, -it's transformed into "user.foo" within Cygwin. This allows Linux-compatible -EA operations and keeps tools like <command>attr</command>, or -<command>setfattr</command> happy. -</para> - -<para><function>chroot</function> is supported since Cygwin 1.1.3. -However, chroot is not a concept known by Windows. This implies some serious -restrictions. First of all, the <function>chroot</function> call isn't a -privileged call. Any user may call it. Second, the chroot environment -isn't safe against native windows processes. Given that, chroot in Cygwin -is only a hack which pretends security where there is none. For that reason -the usage of chroot is discouraged. -</para> -</sect2> - -<sect2 id="ov-hi-textvsbinary"><title>Text Mode vs. Binary Mode</title> -<para>It is often important that files created by native Windows -applications be interoperable with Cygwin applications. For example, a -file created by a native Windows text editor should be readable by a -Cygwin application, and vice versa.</para> - -<para>Unfortunately, UNIX and Win32 have different end-of-line -conventions in text files. A UNIX text file will have a single newline -character (LF) whereas a Win32 text file will instead use a two -character sequence (CR+LF). Consequently, the two character sequence -must be translated on the fly by Cygwin into a single character newline -when reading in text mode.</para> - -<para>This solution addresses the newline interoperability concern at -the expense of violating the POSIX requirement that text and binary mode -be identical. Consequently, processes that attempt to lseek through -text files can no longer rely on the number of bytes read to be an -accurate indicator of position within the file. For this reason, Cygwin -allows you to choose the mode in which a file is read in several ways.</para> -</sect2> - -<sect2 id="ov-hi-ansiclib"><title>ANSI C Library</title> -<para>We chose to include Red Hat's own existing ANSI C library -"newlib" as part of the library, rather than write all of the lib C -and math calls from scratch. Newlib is a BSD-derived ANSI C library, -previously only used by cross-compilers for embedded systems -development. Other functions, which are not supported by newlib have -been added to the Cygwin sources using BSD implementations as much as -possible.</para> - -<para>The reuse of existing free implementations of such things -as the glob, regexp, and getopt libraries saved us considerable -effort. In addition, Cygwin uses Doug Lea's free malloc -implementation that successfully balances speed and compactness. The -library accesses the malloc calls via an exported function pointer. -This makes it possible for a Cygwin process to provide its own -malloc if it so desires.</para> -</sect2> - -<sect2 id="ov-hi-process"><title>Process Creation</title> -<para>The <function>fork</function> call in Cygwin is particularly interesting -because it does not map well on top of the Win32 API. This makes it very -difficult to implement correctly. Currently, the Cygwin fork is a -non-copy-on-write implementation similar to what was present in early -flavors of UNIX.</para> - -<para>The first thing that happens when a parent process -forks a child process is that the parent initializes a space in the -Cygwin process table for the child. It then creates a suspended -child process using the Win32 CreateProcess call. Next, the parent -process calls setjmp to save its own context and sets a pointer to -this in a Cygwin shared memory area (shared among all Cygwin -tasks). It then fills in the child's .data and .bss sections by -copying from its own address space into the suspended child's address -space. After the child's address space is initialized, the child is -run while the parent waits on a mutex. The child discovers it has -been forked and longjumps using the saved jump buffer. The child then -sets the mutex the parent is waiting on and blocks on another mutex. -This is the signal for the parent to copy its stack and heap into the -child, after which it releases the mutex the child is waiting on and -returns from the fork call. Finally, the child wakes from blocking on -the last mutex, recreates any memory-mapped areas passed to it via the -shared area, and returns from fork itself.</para> - -<para>While we have some -ideas as to how to speed up our fork implementation by reducing the -number of context switches between the parent and child process, fork -will almost certainly always be inefficient under Win32. Fortunately, -in most circumstances the spawn family of calls provided by Cygwin -can be substituted for a fork/exec pair with only a little effort. -These calls map cleanly on top of the Win32 API. As a result, they -are much more efficient. Changing the compiler's driver program to -call spawn instead of fork was a trivial change and increased -compilation speeds by twenty to thirty percent in our -tests.</para> - -<para>However, spawn and exec present their own set of -difficulties. Because there is no way to do an actual exec under -Win32, Cygwin has to invent its own Process IDs (PIDs). As a -result, when a process performs multiple exec calls, there will be -multiple Windows PIDs associated with a single Cygwin PID. In some -cases, stubs of each of these Win32 processes may linger, waiting for -their exec'd Cygwin process to exit.</para> -</sect2> - -<sect3 id='ov-hi-process-problems'> -<title>Problems with process creation</title> - -<para>The semantics of <literal>fork</literal> require that a forked -child process have <emphasis>exactly</emphasis> the same address -space layout as its parent. However, Windows provides no native -support for cloning address space between processes and several -features actively undermine a reliable <literal>fork</literal> -implementation. Three issues are especially prevalent:</para> - -<para><itemizedlist> -<listitem>DLL base address collisions. Unlike *nix shared -libraries, which use "position-independent code", Windows shared -libraries assume a fixed base address. Whenever the hard-wired -address ranges of two DLLs collide (which occurs quite often), the -Windows loader must "rebase" one of them to a different -address. However, it may not resolve collisions consistently, and -may rebase a different dll and/or move it to a different address -every time. Cygwin can usually compensate for this effect when it -involves libraries opened dynamically, but collisions among -statically-linked dlls (dependencies known at compile time) are -resolved before <literal>cygwin1.dll</literal> initializes and -cannot be fixed afterward. This problem can only be solved by -removing the base address conflicts which cause the problem, -usually using the <literal>rebaseall</literal> tool.</listitem> - -<listitem>Address space layout randomization (ASLR). Starting with -Vista, Windows implements ASLR, which means that thread stacks, -heap, memory-mapped files, and statically-linked dlls are placed -at different (random) locations in each process. This behaviour -interferes with a proper <literal>fork</literal>, and if an -unmovable object (process heap or system dll) ends up at the wrong -location, Cygwin can do nothing to compensate (though it will -retry a few times automatically).</listitem> - -<listitem>DLL injection by -<ulink url="http://cygwin.com/faq/faq.html#faq.using.bloda"> -BLODA</ulink>. Badly-behaved applications which -inject dlls into other processes often manage to clobber important -sections of the child's address space, leading to base address -collisions which rebasing cannot fix. The only way to resolve this -problem is to remove (usually uninstall) the offending app. See -<xref linkend="cygwinenv-implemented-options"></xref> for the -<literal>detect_bloda</literal> option, which may be able to identify the -BLODA.</listitem></itemizedlist></para> - -<para>In summary, current Windows implementations make it -impossible to implement a perfectly reliable fork, and occasional -fork failures are inevitable. -</para> - -</sect3> - -<sect2 id="ov-hi-signals"><title>Signals</title> -<para>When -a Cygwin process starts, the library starts a secondary thread for -use in signal handling. This thread waits for Windows events used to -pass signals to the process. When a process notices it has a signal, -it scans its signal bitmask and handles the signal in the appropriate -fashion.</para> - -<para>Several complications in the implementation arise from the -fact that the signal handler operates in the same address space as the -executing program. The immediate consequence is that Cygwin system -functions are interruptible unless special care is taken to avoid -this. We go to some lengths to prevent the sig_send function that -sends signals from being interrupted. In the case of a process -sending a signal to another process, we place a mutex around sig_send -such that sig_send will not be interrupted until it has completely -finished sending the signal.</para> - -<para>In the case of a process sending -itself a signal, we use a separate semaphore/event pair instead of the -mutex. sig_send starts by resetting the event and incrementing the -semaphore that flags the signal handler to process the signal. After -the signal is processed, the signal handler signals the event that it -is done. This process keeps intraprocess signals synchronous, as -required by POSIX.</para> - -<para>Most standard UNIX signals are provided. Job -control works as expected in shells that support -it.</para> -</sect2> - -<sect2 id="ov-hi-sockets"><title>Sockets</title> -<para>Socket-related calls in Cygwin basically call the functions by the -same name in Winsock, Microsoft's implementation of Berkeley sockets, but -with lots of tweaks. All sockets are non-blocking under the hood to allow -to interrupt blocking calls by POSIX signals. Additional bookkeeping is -necessary to implement correct socket sharing POSIX semantics and especially -for the select call. Some socket-related functions are not implemented at -all in Winsock, as, for example, socketpair. Starting with Windows Vista, -Microsoft removed the legacy calls <function>rcmd(3)</function>, -<function>rexec(3)</function> and <function>rresvport(3)</function>. -Recent versions of Cygwin now implement all these calls internally.</para> - -<para>An especially troublesome feature of Winsock is that it must be -initialized before the first socket function is called. As a result, Cygwin -has to perform this initialization on the fly, as soon as the first -socket-related function is called by the application. In order to support -sockets across fork calls, child processes initialize Winsock if any -inherited file descriptor is a socket.</para> - -<para>AF_UNIX (AF_LOCAL) sockets are not available in Winsock. They are -implemented in Cygwin by using local AF_INET sockets instead. This is -completely transparent to the application. Cygwin's implementation also -supports the getpeereid BSD extension. However, Cygwin does not yet support -descriptor passing.</para> - -<para>IPv6 is supported beginning with Cygwin release 1.7.0. This -support is dependent, however, on the availability of the Windows IPv6 -stack. The IPv6 stack was "experimental", i.e. not feature complete in -Windows 2003 and earlier. Full IPv6 support became available starting -with Windows Vista and Windows Server 2008. Cygwin does not depend on -the underlying OS for the (newly implemented) <function>getaddrinfo</function> -and <function>getnameinfo</function> functions. Cygwin 1.7.0 adds -replacement functions which implement the full functionality for IPv4.</para> - -</sect2> - -<sect2 id="ov-hi-select"><title>Select</title> -<para>The UNIX <function>select</function> function is another -call that does not map cleanly on top of the Win32 API. Much to our -dismay, we discovered that the Win32 select in Winsock only worked on -socket handles. Our implementation allows select to function normally -when given different types of file descriptors (sockets, pipes, -handles, and a custom /dev/windows Windows messages -pseudo-device).</para> - -<para>Upon entry into the select function, the first -operation is to sort the file descriptors into the different types. -There are then two cases to consider. The simple case is when at -least one file descriptor is a type that is always known to be ready -(such as a disk file). In that case, select returns immediately as -soon as it has polled each of the other types to see if they are -ready. The more complex case involves waiting for socket or pipe file -descriptors to be ready. This is accomplished by the main thread -suspending itself, after starting one thread for each type of file -descriptor present. Each thread polls the file descriptors of its -respective type with the appropriate Win32 API call. As soon as a -thread identifies a ready descriptor, that thread signals the main -thread to wake up. This case is now the same as the first one since -we know at least one descriptor is ready. So select returns, after -polling all of the file descriptors one last time.</para> -</sect2> -</sect1> - |