On 12/29/18 10:02 AM, Willy Tarreau wrote: > Ingo rightfully asked for a bit more documentation in the nolibc header, > so this patch adds some explanation about its purpose, how it's made, and > how to use it. > > Cc: Ingo Molnar <mi...@kernel.org> > Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Cc: Randy Dunlap <rdun...@infradead.org> > Signed-off-by: Willy Tarreau <w...@1wt.eu>
Reviewed-by: Randy Dunlap <rdun...@infradead.org> Thanks. > --- > tools/testing/selftests/rcutorture/bin/nolibc.h | 90 > +++++++++++++++++++++---- > 1 file changed, 78 insertions(+), 12 deletions(-) > > diff --git a/tools/testing/selftests/rcutorture/bin/nolibc.h > b/tools/testing/selftests/rcutorture/bin/nolibc.h > index 985364c..6643ba9 100644 > --- a/tools/testing/selftests/rcutorture/bin/nolibc.h > +++ b/tools/testing/selftests/rcutorture/bin/nolibc.h > @@ -3,6 +3,84 @@ > * Copyright (C) 2017-2018 Willy Tarreau <w...@1wt.eu> > */ > > +/* > + * This file is designed to be used as a libc alternative for minimal > programs > + * with very limited requirements. It consists of a small number of syscall > and > + * type definitions, and the minimal startup code needed to call main(). > + * All syscalls are declared as static functions so that they can be > optimized > + * away by the compiler when not used. > + * > + * Syscalls are split into 3 levels: > + * - the lower level is the arch-specific syscall() definition, consisting > in > + * assembly code in compound expressions. These are called my_syscall0() > to > + * my_syscall6() depending on the number of arguments. The MIPS > + * implementation is limited to 5 arguments. All input arguments are cast > + * to a long stored in a register. These expressions always return the > + * syscall's return value as a signed long value which is often either a > + * pointer or the negated errno value. > + * > + * - the second level is mostly architecture-independent. It is made of > + * static functions called sys_<name>() which rely on my_syscallN() > + * depending on the syscall definition. These functions are responsible > + * for exposing the appropriate types for the syscall arguments (int, > + * pointers, etc) and for setting the appropriate return type (often > int). > + * A few of them are architecture-specific because the syscalls are not > all > + * mapped exactly the same among architectures. For example, some archs > do > + * not implement select() and need pselect6() instead, so the > sys_select() > + * function will have to abstract this. > + * > + * - the third level is the libc call definition. It exposes the lower raw > + * sys_<name>() calls in a way that looks like what a libc usually does, > + * takes care of specific input values, and of setting errno upon error. > + * There can be minor variations compared to standard libc calls. For > + * example the open() call always takes 3 args here. > + * > + * The errno variable is declared static and unused. This way it can be > + * optimized away if not used. However this means that a program made of > + * multiple C files may observe different errno values (one per C file). For > + * the type of programs this project targets it usually is not a problem. The > + * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO > + * macro, in which case the errno value will never be assigned. > + * > + * Some stdint-like integer types are defined. These are valid on all > currently > + * supported architectures, because signs are enforced, ints are assumed to > be > + * 32 bits, longs the size of a pointer and long long 64 bits. If more > + * architectures have to be supported, this may need to be adapted. > + * > + * Some macro definitions like the O_* values passed to open(), and some > + * structures like the sys_stat struct depend on the architecture. > + * > + * The definitions start with the architecture-specific parts, which are > picked > + * based on what the compiler knows about the target architecture, and are > + * completed with the generic code. Since it is the compiler which sets the > + * target architecture, cross-compiling normally works out of the box without > + * having to specify anything. > + * > + * Finally some very common libc-level functions are provided. It is the case > + * for a few functions usually found in string.h, ctype.h, or stdlib.h. > Nothing > + * is currently provided regarding stdio emulation. > + * > + * The macro NOLIBC is always defined, so that it is possible for a program > to > + * check this macro to know if it is being built against and decide to > disable > + * some features or simply not to include some standard libc files. > + * > + * Ideally this file should be split in multiple files for easier long term > + * maintenance, but provided as a single file as it is now, it's quite > + * convenient to use. Maybe some variations involving a set of includes at > the > + * top could work. > + * > + * A simple static executable may be built this way : > + * $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \ > + * -static -include nolibc.h -lgcc -o hello hello.c > + * > + * A very useful calling convention table may be found here : > + * http://man7.org/linux/man-pages/man2/syscall.2.html > + * > + * This doc is quite convenient though not necessarily up to date : > + * https://w3challs.com/syscalls/ > + * > + */ > + > /* some archs (at least aarch64) don't expose the regular syscalls anymore by > * default, either because they have an "_at" replacement, or because there > are > * more modern alternatives. For now we'd rather still use them. > @@ -19,18 +97,6 @@ > > #define NOLIBC > > -/* Build a static executable this way : > - * $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \ > - * -static -include nolibc.h -lgcc -o hello hello.c > - * > - * Useful calling convention table found here : > - * http://man7.org/linux/man-pages/man2/syscall.2.html > - * > - * This doc is even better : > - * https://w3challs.com/syscalls/ > - */ > - > - > /* this way it will be removed if unused */ > static int errno; > > -- ~Randy
