inability to use realpath in chroot/container without procfs access and at early boot prior to mount of /proc has been an ongoing issue, and it turns out realpath was one of the last remaining interfaces that needed procfs for its core functionality. during investigation while reimplementing, it was determined that there were also serious problems with the procfs-based implementation. most seriously it was unsafe on pre-O_PATH kernels, and unlike other places where O_PATH was used, the unsafety was hard or impossible to fix because O_NOFOLLOW can't be used (since the whole purpose was to follow symlinks). the new implementation is a direct one, performing readlink on each path component to resolve it. an explicit stack, as opposed to recursion, is used to represent the remaining components to be processed. the stack starts out holding just the input string, and reading a link pushes the link contents onto the stack. unlike many other implementations, this one does not call getcwd initially for relative pathnames. instead it accumulates initial .. components to be applied to the working directory if the result is still a relative path. this avoids calling getcwd (which may fail) at all when symlink traversal will eventually yield an absolute path. it also doesn't use any form of stat operation; instead it arranges for readlink to tell it when a non-directory is used in a context where a directory is needed. this minimizes the number of syscalls needed, avoids accessing inodes when the directory table suffices, and reduces the amount of code pulled in for static linking.
syscall.h was chosen as the header to declare it, since its intended usage is alongside syscalls as a fallback for operations the direct syscall does not support.
open is handled specially because it is used from so many places, in so many variants (2 or 3 arguments, setting errno or not, and cancellable or not). trying to do it as a function would not only increase bloat, but would also risk subtle breakage. this is the first step towards supporting "new" archs where linux lacks "old" syscalls.
rather than allocating a PATH_MAX-sized buffer when the caller does not provide an output buffer, work first with a PATH_MAX-sized temp buffer with automatic storage, and either copy it to the caller's buffer or strdup it on success. this not only avoids massive memory waste, but also avoids pulling in free (and thus the full malloc implementation) unnecessarily in static programs.
this avoids failure if the file is not readable and avoids odd behavior for device nodes, etc. on old kernels that lack O_PATH, the old behavior (O_RDONLY) will naturally happen as the fallback.
I intend to add more Linux workarounds that depend on using these pathnames, and some of them will be in "syscall" functions that, from an anti-bloat standpoint, should not depend on the whole snprintf framework.
to deal with the fact that the public headers may be used with pre-c99 compilers, __restrict is used in place of restrict, and defined appropriately for any supported compiler. we also avoid the form [restrict] since older versions of gcc rejected it due to a bug in the original c99 standard, and instead use the form *restrict.