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Re: [Qemu-devel] [PATCH] cputlb: don't cpu_abort() if guest tries to exe
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] cputlb: don't cpu_abort() if guest tries to execute outside RAM or RAM |
Date: |
Tue, 28 Jun 2016 19:49:53 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 |
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 <address@hidden> 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 <address@hidden>
>> ---
>> 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
>
>