| Message ID | 20230731080317.112658-4-akihiko.odaki@daynix.com |
|---|---|
| State | New |
| Headers | show |
| Series | linux-user: brk/mmap fixes | expand |
On Mon, 31 Jul 2023 at 09:04, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: > > MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without > concerning that the new mapping overwrites something else. MAP_FIXED_NOREPLACE only came in with Linux 4.17. So I think we still need to handle the "mapped address is not the one we asked for" error condition, because it can happen on older host kernels that ignore the MAP_FIXED_NOREPLACE flag. thanks -- PMM
On 2023/07/31 20:44, Peter Maydell wrote: > On Mon, 31 Jul 2023 at 09:04, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >> >> MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without >> concerning that the new mapping overwrites something else. > > MAP_FIXED_NOREPLACE only came in with Linux 4.17. So > I think we still need to handle the "mapped address > is not the one we asked for" error condition, because > it can happen on older host kernels that ignore the > MAP_FIXED_NOREPLACE flag. > > thanks > -- PMM MAP_FIXED_NOREPLACE is substituted with MAP_FIXED before passing to the host with patch 1. The NOREPLACE constraint is still ensured by inspecting the guest page table. Regards, Akihiko Odaki
On Mon, 31 Jul 2023 at 14:32, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: > > On 2023/07/31 20:44, Peter Maydell wrote: > > On Mon, 31 Jul 2023 at 09:04, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: > >> > >> MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without > >> concerning that the new mapping overwrites something else. > > > > MAP_FIXED_NOREPLACE only came in with Linux 4.17. So > > I think we still need to handle the "mapped address > > is not the one we asked for" error condition, because > > it can happen on older host kernels that ignore the > > MAP_FIXED_NOREPLACE flag. > MAP_FIXED_NOREPLACE is substituted with MAP_FIXED before passing to the > host with patch 1. The NOREPLACE constraint is still ensured by > inspecting the guest page table. Oh, I see, this patch is a call to target_mmap(), not host mmap(). Sorry, I misread that. thanks -- PMM
On 7/31/23 06:32, Akihiko Odaki wrote: > On 2023/07/31 20:44, Peter Maydell wrote: >> On Mon, 31 Jul 2023 at 09:04, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>> >>> MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without >>> concerning that the new mapping overwrites something else. >> >> MAP_FIXED_NOREPLACE only came in with Linux 4.17. So >> I think we still need to handle the "mapped address >> is not the one we asked for" error condition, because >> it can happen on older host kernels that ignore the >> MAP_FIXED_NOREPLACE flag. >> >> thanks >> -- PMM > > MAP_FIXED_NOREPLACE is substituted with MAP_FIXED before passing to the host with patch 1. > The NOREPLACE constraint is still ensured by inspecting the guest page table. Won't work for 64-bit guests. r~
diff --git a/linux-user/syscall.c b/linux-user/syscall.c index b9d2ec02f9..ac429a185a 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -854,17 +854,12 @@ abi_long do_brk(abi_ulong brk_val) return target_brk; } - /* We need to allocate more memory after the brk... Note that - * we don't use MAP_FIXED because that will map over the top of - * any existing mapping (like the one with the host libc or qemu - * itself); instead we treat "mapped but at wrong address" as - * a failure and unmap again. - */ if (new_host_brk_page > brk_page) { new_alloc_size = new_host_brk_page - brk_page; mapped_addr = target_mmap(brk_page, new_alloc_size, - PROT_READ|PROT_WRITE, - MAP_ANON|MAP_PRIVATE, 0, 0); + PROT_READ | PROT_WRITE, + MAP_FIXED_NOREPLACE | MAP_ANON | MAP_PRIVATE, + 0, 0); } else { new_alloc_size = 0; mapped_addr = brk_page; @@ -883,12 +878,6 @@ abi_long do_brk(abi_ulong brk_val) target_brk = brk_val; brk_page = new_host_brk_page; return target_brk; - } else if (mapped_addr != -1) { - /* Mapped but at wrong address, meaning there wasn't actually - * enough space for this brk. - */ - target_munmap(mapped_addr, new_alloc_size); - mapped_addr = -1; } #if defined(TARGET_ALPHA)
MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without concerning that the new mapping overwrites something else. Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> --- linux-user/syscall.c | 17 +++-------------- 1 file changed, 3 insertions(+), 14 deletions(-)