Re: [PATCH v3 02/13] tcg: Fix error reporting on mprotect() failure in tcg_region_init()

[Markus Armbruster <armbru@redhat.com>]

Philippe Mathieu-Daudé <philmd@linaro.org> writes:

> On 23/9/25 11:09, Markus Armbruster wrote:
>> tcg_region_init() calls one of qemu_mprotect_rwx(),
>> qemu_mprotect_rw(), and mprotect(), then reports failure with
>> error_setg_errno(&error_fatal, errno, ...).
>>
>> The use of &error_fatal is undesirable.  qapi/error.h advises:
>>
>>   * Please don't error_setg(&error_fatal, ...), use error_report() and
>>   * exit(), because that's more obvious.
>>
>> The use of errno is wrong.  qemu_mprotect_rwx() and qemu_mprotect_rw()
>> wrap around qemu_mprotect__osdep().  qemu_mprotect__osdep() calls
>> mprotect() on POSIX, VirtualProtect() on Windows, and reports failure
>> with error_report().  VirtualProtect() doesn't set errno.  mprotect()
>> does, but error_report() may clobber it.
>>
>> Fix tcg_region_init() to report errors only when it calls mprotect(),
>> and rely on qemu_mprotect_rwx()'s and qemu_mprotect_rw()'s error
>> reporting otherwise.  Use error_report(), not error_setg().
>>
>> Fixes: 22c6a9938f75 (tcg: Merge buffer protection and guard page protection)
>> Fixes: 6bc144237a85 (tcg: Use Error with alloc_code_gen_buffer)
>> Cc: Richard Henderson <richard.henderson@linaro.org>
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>   tcg/region.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>> diff --git a/tcg/region.c b/tcg/region.c
>> index 7ea0b37a84..2181267e48 100644
>> --- a/tcg/region.c
>> +++ b/tcg/region.c
>> @@ -832,13 +832,16 @@ void tcg_region_init(size_t tb_size, int splitwx, unsigned max_threads)
>>              } else {
>>  #ifdef CONFIG_POSIX
>>                  rc = mprotect(start, end - start, need_prot);
>> +                if (rc) {
>> +                    error_report("mprotect of jit buffer: %s",
>> +                                 strerror(errno));
>
> I'm not keen on handling errors differently in the same function.
>
> qemu_mprotect_rwx() and qemu_mprotect_rw() already print the error.

Yes: they call qemu_mprotect__osdep(), which uses error_report().

> Why not add qemu_mprotect() as a simple qemu_mprotect__osdep() alias,
> then call it here, also covering the non-POSIX case?
> (Question for Richard, after looking at commits 22c6a9938f7 and more
>  importantly 97a83753c9 -- wondering about WoA).

There is no commit 97a83753c9.  Do you mean a97a83753c9?

I'd like to merge this commit as is.  It's a minimal fix, and it's been
reviewed.  We can always improve on top.

>> +                }
>>  #else
>>                  g_assert_not_reached();
>>  #endif
>>              }
>>              if (rc) {
>> -                error_setg_errno(&error_fatal, errno,
>> -                                 "mprotect of jit buffer");
>> +                exit(1);
>>              }
>>           }
>>           if (have_prot != 0) {