From b7da02dad0f25928e688a5dce74a0f10fa120594 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Wed, 10 Mar 2021 17:41:26 -0600 Subject: [PATCH] tcg: When allocating for !splitwx, begin with PROT_NONE 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 Reviewed-by: Luis Pires Signed-off-by: Richard Henderson --- tcg/region.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/tcg/region.c b/tcg/region.c index 42d3fa9ba1..00b0c3b091 100644 --- a/tcg/region.c +++ b/tcg/region.c @@ -770,12 +770,15 @@ 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; @@ -906,11 +909,7 @@ void tcg_region_init(size_t tb_size, int splitwx, unsigned max_cpus) } } 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); } }