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

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/06/2016 17:42, Peter Maydell wrote:
> Ping for review?

The patch is trivial, the hard part was coming up with the message for
the user. :)  Go ahead!

Paolo

> 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
> 
>