linux-dev/arch/um/os-Linux/skas/process.c, branch linus/master

um: Fix stack pointer alignment

2021-06-17T20:08:31Z

GCC assumes that stack is aligned to 16-byte on call sites [1]. Since GCC 8, GCC began using 16-byte aligned SSE instructions to implement assignments to structs on stack. When CC_OPTIMIZE_FOR_PERFORMANCE is enabled, this affects os-Linux/sigio.c, write_sigio_thread: struct pollfds *fds, tmp; tmp = current_poll; Note that struct pollfds is exactly 16 bytes in size. GCC 8+ generates assembly similar to: movdqa (%rdi),%xmm0 movaps %xmm0,-0x50(%rbp) This is an issue, because movaps will #GP if -0x50(%rbp) is not aligned to 16 bytes [2], and how rbp gets assigned to is via glibc clone thread_start, then function prologue, going though execution trace similar to (showing only relevant instructions): sub $0x10,%rsi mov %rcx,0x8(%rsi) mov %rdi,(%rsi) syscall pop %rax pop %rdi callq *%rax push %rbp mov %rsp,%rbp The stack pointer always points to the topmost element on stack, rather then the space right above the topmost. On push, the pointer decrements first before writing to the memory pointed to by it. Therefore, there is no need to have the stack pointer pointer always point to valid memory unless the stack is poped; so the `- sizeof(void *)` in the code is unnecessary. On the other hand, glibc reserves the 16 bytes it needs on stack and pops itself, so by the call instruction the stack pointer is exactly the caller-supplied sp. It then push the 16 bytes of the return address and the saved stack pointer, so the base pointer will be 16-byte aligned if and only if the caller supplied sp is 16-byte aligned. Therefore, the caller must supply a 16-byte aligned pointer, which `stack + UM_KERN_PAGE_SIZE` already satisfies. On a side note, musl is unaffected by this issue because it forces 16 byte alignment via `and $-16,%rsi` in its clone wrapper. Similarly, glibc i386 is also unaffected because it has `andl $0xfffffff0, %ecx`. To reproduce this bug, enable CONFIG_UML_RTC and CC_OPTIMIZE_FOR_PERFORMANCE. uml_rtc will call add_sigio_fd which will then cause write_sigio_thread to either go into segfault loop or panic with "Segfault with no mm". Similarly, signal stacks will be aligned by the host kernel upon signal delivery. `- sizeof(void *)` to sigaltstack is unconventional and extraneous. On a related note, initialization of longjmp buffers do require `- sizeof(void *)`. This is to account for the return address that would have been pushed to the stack at the call site. The reason for uml to respect 16-byte alignment, rather than telling GCC to assume 8-byte alignment like the host kernel since commit d9b0cde91c60 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported"), is because uml links against libc. There is no reason to assume libc is also compiled with that flag and assumes 8-byte alignment rather than 16-byte. [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838 [2] https://c9x.me/x86/html/file_module_x86_id_180.html Signed-off-by: YiFei Zhu Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reviewed-by: Johannes Berg Signed-off-by: Richard Weinberger

um: remove process stub VMA

2021-02-12T20:37:38Z

This mostly reverts the old commit 3963333fe676 ("uml: cover stubs with a VMA") which had added a VMA to the existing PTEs. However, there's no real reason to have the PTEs in the first place and the VMA cannot be 'fixed' in place, which leads to bugs that userspace could try to unmap them and be forcefully killed, or such. Also, there's a bit of an ugly hole in userspace's address space. Simplify all this: just install the stub code/page at the top of the (inner) address space, i.e. put it just above TASK_SIZE. The pages are simply hard-coded to be mapped in the userspace process we use to implement an mm context, and they're out of reach of the inner mmap/munmap/mprotect etc. since they're above TASK_SIZE. Getting rid of the VMA also makes vma_merge() no longer hit one of the VM_WARN_ON()s there because we installed a VMA while the code assumes the stack VMA is the first one. It also removes a lockdep warning about mmap_sem usage since we no longer have uml_setup_stubs() and thus no longer need to do any manipulation that would require mmap_sem in activate_mm(). Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger

um: separate child and parent errors in clone stub

2021-02-12T20:34:33Z

If the two are mixed up, then it looks as though the parent returned an error if the child failed (before) the mmap(), and then the resulting process never gets killed. Fix this by splitting the child and parent errors, reporting and using them appropriately. Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger

um: defer killing userspace on page table update failures

2021-02-12T20:32:04Z

In some cases we can get to fix_range_common() with mmap_sem held, and in others we get there without it being held. For example, we get there with it held from sys_mprotect(), and without it held from fork_handler(). Avoid any issues in this and simply defer killing the task until it runs the next time. Do it on the mm so that another task that shares the same mm can't continue running afterwards. Cc: stable@vger.kernel.org Fixes: 468f65976a8d ("um: Fix hung task in fix_range_common()") Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger

um: print register names in wait_for_stub

2021-02-12T20:30:19Z

Since we're basically debugging the userspace (it runs in ptrace) it's useful to dump out the registers - but they're not readable, so if something goes wrong it's hard to say what. Print the names of registers in the register dump so it's easier to look at. Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger

um: Fetch registers only for signals which need them

2020-12-13T21:38:06Z

UML userspace fetches siginfo and passes it to signal handlers in UML. This is needed only for some of the signals, because key handlers like SIGIO make no use of this variable. Signed-off-by: Anton Ivanov Signed-off-by: Richard Weinberger

um: Add SPDX headers for files in arch/um/os-Linux

2019-09-15T19:37:17Z

Convert files to use SPDX header. All files are licensed under the GPLv2. Signed-off-by: Alex Dewar Signed-off-by: Richard Weinberger

um: Implement TRACE_IRQFLAGS_SUPPORT

2019-09-15T19:37:11Z

UML enables TRACE_IRQFLAGS_SUPPORT but doesn't actually implement it. It seems to have been added for lockdep support, but that can't actually really work well without IRQ flags tracing, as is also very noisily reported when enabling CONFIG_DEBUG_LOCKDEP. Implement it now. Fixes: 711553efa5b8 ("[PATCH] uml: declare in Kconfig our partial LOCKDEP support") Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger

um: Give start_idle_thread() a return code

2018-10-29T21:23:11Z

Fixes: arch/um/os-Linux/skas/process.c:613:1: warning: control reaches end of non-void function [-Wreturn-type] longjmp() never returns but gcc still warns that the end of the function can be reached. Add a return code and debug aid to detect this impossible case. Signed-off-by: Richard Weinberger

um: Fix FP register size for XSTATE/XSAVE

2017-09-13T20:24:38Z

Hard code max size. Taken from https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/common/x86-xstate.h Signed-off-by: Thomas Meyer Signed-off-by: Richard Weinberger