Date: Thu, 11 Apr 2019 16:25:50 +0200 From: Norbert Lange <nolange79@...il.com> To: musl@...ts.openwall.com Subject: use of varargs in open and various functions Hello, I had some dealings with software that forwards arguments from vararg functions, the kind with optional but known fixed type. open, fcntl, ioctl are some examples. What happens in musl is that the optional argument is optionally or always retrieved (fcntl). I think this is pretty much pointless, the only reason to use varags would be: 1. varying argument types (plainly not possibly to replace with direct arguments) 2. different parameter passing than direct arguments (ABI compatibility) 3. accessing parameters the caller has not provided. 1. disqualifies functions like printf which I am not planning to touch. 2. would need more tests, but I expect this to be true for all modern architectures, atleast its true for most of them. 3. thats a thing for open where the 3rd argument is only accessed if certain flags are set, but most other functions in musl seem to always access "optional" arguments. reading theses non-existing arguments could trigger tools like valgrind, but other than that it should not be an issue aslong a couple bytes of stack is available. I tested code calling the functions, I believe calls are identical for varags and normal functions, for all architectures that are supported by musl.  (sidenote: clang generates absolutely awful code for those varag functions). So, is there any reason not to make this the default for musl (adding a fallback option via a define)? I am running a few tools (bash, nano) compiled with the applied patch and so far I have no adverse effects, it should not affect ABI for machines satisfying 2. and it gets rid of some wacky code that in the end just passes in another register (like open). Further I would like to add that Torvalds shares a similar view . Kind regards, Norbert  https://godbolt.org/z/asBT92  https://lkml.org/lkml/2014/10/30/812 PS. why is this a thing in open: int fd = __sys_open_cp(filename, flags, mode); if (fd>=0 && (flags & O_CLOEXEC)) __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC); Is this to support old kernels? I thought O_CLOEXEC is used to fight races during a fork, so if its not supported by the kernel it wont do alot. View attachment "lessvarargs.patch" of type "text/x-patch" (5933 bytes)
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