Re: [Qemu-devel] [PATCH] cputlb: don't cpu_abort() if guest tries to execute outside RAM or RAM

[Peter Maydell <peter.maydell@linaro.org>]

Ping for review?

thanks
-- PMM

On 20 June 2016 at 18:07, Peter Maydell <peter.maydell@linaro.org> wrote:
> In get_page_addr_code(), if the guest program counter turns out not to
> be in ROM or RAM, we can't handle executing from it, and we call
> cpu_abort(). This results in the message
>   qemu: fatal: Trying to execute code outside RAM or ROM at 0x08000000
> followed by a guest register dump, and then QEMU dumps core.
>
> This situation happens in one of two cases:
>  (1) a guest kernel bug, where it jumped off into nowhere
>  (2) a user command line mistake, where they tried to run an image for
>      board A on a QEMU model of board B, or where they didn't provide
>      an image at all, and QEMU executed through a ROM or RAM full of
>      NOP instructions and then fell off the end
>
> In either case, a core dump of QEMU itself is entirely useless, and
> only confuses users into thinking that this is a bug in QEMU rather
> than a bug in the guest or a problem with their command line. (This
> is a variation on the general idea that we shouldn't assert() on
> something the user can accidentally provoke.)
>
> Replace the cpu_abort() with something that explains the situation
> a bit better and exits QEMU without dumping core.
>
> (See LP:1062220 for several examples of confused users.)
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> I've been meaning to do this for a while now...hopefully the
> expanded error message should reduce user confusion.
>
>  cputlb.c | 39 +++++++++++++++++++++++++++++++++++++--
>  1 file changed, 37 insertions(+), 2 deletions(-)
>
> diff --git a/cputlb.c b/cputlb.c
> index 23c9b91..079e497 100644
> --- a/cputlb.c
> +++ b/cputlb.c
> @@ -30,6 +30,8 @@
>  #include "exec/ram_addr.h"
>  #include "exec/exec-all.h"
>  #include "tcg/tcg.h"
> +#include "qemu/error-report.h"
> +#include "exec/log.h"
>
>  /* DEBUG defines, enable DEBUG_TLB_LOG to log to the CPU_LOG_MMU target */
>  /* #define DEBUG_TLB */
> @@ -427,6 +429,39 @@ void tlb_set_page(CPUState *cpu, target_ulong vaddr,
>                              prot, mmu_idx, size);
>  }
>
> +static void report_bad_exec(CPUState *cpu, target_ulong addr)
> +{
> +    /* Accidentally executing outside RAM or ROM is quite common for
> +     * several user-error situations, so report it in a way that
> +     * makes it clear that this isn't a QEMU bug and provide suggestions
> +     * about what a user could do to fix things.
> +     */
> +    error_report("Trying to execute code outside RAM or ROM at 0x"
> +                 TARGET_FMT_lx, addr);
> +    error_printf("This usually means one of the following happened:\n\n"
> +                 "(1) You told QEMU to execute a kernel for the wrong machine "
> +                 "type, and it crashed on startup (eg trying to run a "
> +                 "raspberry pi kernel on a versatilepb QEMU machine)\n"
> +                 "(2) You didn't give QEMU a kernel or BIOS filename at all, "
> +                 "and QEMU executed a ROM full of no-op instructions until "
> +                 "it fell off the end\n"
> +                 "(3) Your guest kernel has a bug and crashed by jumping "
> +                 "off into nowhere\n\n"
> +                 "This is almost always one of the first two, so check your "
> +                 "command line and that you are using the right type of kernel "
> +                 "for this machine.\n"
> +                 "If you think option (3) is likely then you can try debugging "
> +                 "your guest with the -d debug options; in particular "
> +                 "-d guest_errors will cause the log to include a dump of the "
> +                 "guest register state at this point.\n\n"
> +                 "Execution cannot continue; stopping here.\n\n");
> +
> +    /* Report also to the logs, with more detail including register dump */
> +    qemu_log_mask(LOG_GUEST_ERROR, "qemu: fatal: Trying to execute code "
> +                  "outside RAM or ROM at 0x" TARGET_FMT_lx "\n", addr);
> +    log_cpu_state_mask(LOG_GUEST_ERROR, cpu, CPU_DUMP_FPU | CPU_DUMP_CCOP);
> +}
> +
>  /* NOTE: this function can trigger an exception */
>  /* NOTE2: the returned address is not exactly the physical address: it
>   * is actually a ram_addr_t (in system mode; the user mode emulation
> @@ -455,8 +490,8 @@ tb_page_addr_t get_page_addr_code(CPUArchState *env1, target_ulong addr)
>          if (cc->do_unassigned_access) {
>              cc->do_unassigned_access(cpu, addr, false, true, 0, 4);
>          } else {
> -            cpu_abort(cpu, "Trying to execute code outside RAM or ROM at 0x"
> -                      TARGET_FMT_lx "\n", addr);
> +            report_bad_exec(cpu, addr);
> +            exit(1);
>          }
>      }
>      p = (void *)((uintptr_t)addr + env1->tlb_table[mmu_idx][page_index].addend);
> --
> 1.9.1