Date: Mon, 1 Jun 2015 11:11:07 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Cc: rob@...dley.net Subject: Moving forward with sh2/nommu [resent to musl list] Here's a summary of the issues we need to work through to get a modern SH2/nommu-targetted musl/toolchain out of the proof-of-concept stage and to the point where it's something people can use roughly 'out of the box': Kernel issues: 1. Kernel should support loading plain ELF directly, unmodified. Right now I'm writing 0x81 to byte 38 of the header to make a "non-FDPIC FDPIC ELF binary", which works, but I have to make a personality() syscall at startup to switch back (this matters to kernel signal handling) and it's just highly inconvenient/ugly. Despite plain ELF being suitable for NOMMU, the loader implementation in binfmt_elf.c depends pretty heavily on MMU. The one in binfmt_elf_fdpic.c can work on either. The easiest way forward is to make it so that binfmt_elf_fdpic.c does not insist on having the FDPIC flags in the ELF header on NOMMU targets (where it won't confict with binfmt_elf.c since that loader isn't usable). 2. Kernel insists on having a stack size set in the PT_GNU_STACK program header; if it's 0 (the default ld produces) then execve fails. It should just provide a default, probably 128k (equal to MMU-ful Linux). 3. Kernel uses the stack for brk too, growing brk from the opposite end. This is horribly buggy/dangerous. Just dummying out brk to always-fail is what should be done, but if it can't be done on the kernel side musl can do it instead (that's what I'm doing now). Unfortunately I suspect fixing this might be controversial since there may be existing binaries using brk that can't fall back to mmap. 4. Syscall trap numbers differ on SH2 vs SH3/4. Presumably the reason is that these two SH2A hardware traps overlap with the syscall range used by SH3/4 ABI: # define TRAP_DIVZERO_ERROR 17 # define TRAP_DIVOVF_ERROR 18 The path forward I'd like to see is deprecating everything but trap numbers 22 and 38, which, as far as I can tell, are safe for both the SH2 and SH3/4 kernel to treat as syscalls. These numbers indicate "6 arguments"; there is no good reason to encode the number of arguments in the trap number, so we might as well just always use the "6 argument" code which is what the variadic syscall() has to use anyway. User code should then aim to use the correct value (22 or 38) for the model it's running on (SH3/4 or SH2) for compatibility with old kernels, but will still run safely on new kernels if it detects wrong. Toolchain issues: 1. We need static-PIE (with or without TEXTRELs) which gcc does not support out of the box. I have complex command lines that produce static-PIE, and I have specfile based recipes to convert a normal toolchain to produce (either optionally or by default) static-PIE, but these recipes conflict with using the same toolchain to build the kernel. If static-PIE were integrated properly upstream that would not be an issue. 2. Neither binutils nor gcc accepts "sh2eb-linux" as a target. Trying to hack it in got me a little-endian toolchain. I'm currently just using "sheb" and -m2 to use sh2 instructions that aren't in sh1. 3. The complex math functions cause ICE in all gcc versions I've tried targetting SH2. For now we can just remove src/complex from musl, but that's a hack. The cause of this bug needs to be found and fixed in GCC. musl issues: 1. We need runtime detection for the right trap number to use for syscalls. Right now I've got the trap numbers hard-coded for SH2 in my local tree. 2. We need additional runtime detection options for atomics: interrupt masking for plain SH2, and the new CAS instruction for SH2J. 3. We need sh/vfork.s since the default vfork.c just uses fork, which won't work. I have a version locally but it doesn't make sense to commit without runtime trap number selection. 4. As long as we're using the FDPIC ELF header flag to get binfmt_elf_fdpic.c to load binaries, the startup code needs to call the personality() syscall to switch back. I have a local hack for doing this in rcrt1.o which is probably not worth upstreaming if we can just make the kernel do it right. 5. The brk workaround I'm doing now can't be upstreamed without a reliable runtime way to distinguish nommu. To put it in malloc.c this would have to be a cross-arch solution. What might make more sense is putting it in syscall_arch.h for sh, where we already have to check for SH2 to determine the right trap number; the inline syscall code can just do if (nr==SYS_brk&&IS_SH2) return 0;
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