Richard Henderson <richard.hender...@linaro.org> writes:
> There's a change in mprotect() behaviour [1] in the latest macOS > on M1 and it's not yet clear if it's going to be fixed by Apple. > > In this case, instead of changing permissions of N guard pages, > we change permissions of N rwx regions. The same number of > syscalls are required either way. > > [1] https://gist.github.com/hikalium/75ae822466ee4da13cbbe486498a191f > > Signed-off-by: Richard Henderson <richard.hender...@linaro.org> > --- > tcg/region.c | 19 +++++++++---------- > 1 file changed, 9 insertions(+), 10 deletions(-) > > diff --git a/tcg/region.c b/tcg/region.c > index 604530b902..5e00db4cfb 100644 > --- a/tcg/region.c > +++ b/tcg/region.c > @@ -765,12 +765,15 @@ static int alloc_code_gen_buffer(size_t size, int > splitwx, Error **errp) > error_free_or_abort(errp); > } > > - prot = PROT_READ | PROT_WRITE | PROT_EXEC; > + /* > + * macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect > + * rejects a permission change from RWX -> NONE when reserving the > + * guard pages later. We can go the other way with the same number > + * of syscalls, so always begin with PROT_NONE. > + */ > + prot = PROT_NONE; > flags = MAP_PRIVATE | MAP_ANONYMOUS; > -#ifdef CONFIG_TCG_INTERPRETER > - /* The tcg interpreter does not need execute permission. */ > - prot = PROT_READ | PROT_WRITE; > -#elif defined(CONFIG_DARWIN) > +#ifdef CONFIG_DARWIN > /* Applicable to both iOS and macOS (Apple Silicon). */ > if (!splitwx) { > flags |= MAP_JIT; > @@ -901,11 +904,7 @@ void tcg_region_init(size_t tb_size, int splitwx, > unsigned max_cpus) > } > } I think the comment in tcg_region_init needs to be updated, currently reads: /* * Set guard pages in the rw buffer, as that's the one into which * buffer overruns could occur. Do not set guard pages in the rx * buffer -- let that one use hugepages throughout. * Work with the page protections set up with the initial mapping. */ need_prot = PAGE_READ | PAGE_WRITE; but now we start with everything at PROT_NONE and are just setting need_prot for the non-guard pages. > if (have_prot != 0) { > - /* > - * macOS 11.2 has a bug (Apple Feedback FB8994773) in which > mprotect > - * rejects a permission change from RWX -> NONE. Guard pages are > - * nice for bug detection but are not essential; ignore any > failure. > - */ > + /* Guard pages are nice for bug detection but are not essential. > */ > (void)qemu_mprotect_none(end, page_size); Isn't the last page already set as PROT_NONE? > } > } -- Alex Bennée
