qemu/accel/tcg
Eric Blake f9919116b8 osdep: Make MIN/MAX evaluate arguments only once
I'm not aware of any immediate bugs in qemu where a second runtime
evaluation of the arguments to MIN() or MAX() causes a problem, but
proactively preventing such abuse is easier than falling prey to an
unintended case down the road.  At any rate, here's the conversation
that sparked the current patch:
https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html

Update the MIN/MAX macros to only evaluate their argument once at
runtime; this uses typeof(1 ? (a) : (b)) to ensure that we are
promoting the temporaries to the same type as the final comparison (we
have to trigger type promotion, as typeof(bitfield) won't compile; and
we can't use typeof((a) + (b)) or even typeof((a) + 0), as some of our
uses of MAX are on void* pointers where such addition is undefined).

However, we are unable to work around gcc refusing to compile ({}) in
a constant context (such as the array length of a static variable),
even when only used in the dead branch of a __builtin_choose_expr(),
so we have to provide a second macro pair MIN_CONST and MAX_CONST for
use when both arguments are known to be compile-time constants and
where the result must also be usable as a constant; this second form
evaluates arguments multiple times but that doesn't matter for
constants.  By using a void expression as the expansion if a
non-constant is presented to this second form, we can enlist the
compiler to ensure the double evaluation is not attempted on
non-constants.

Alas, as both macros now rely on compiler intrinsics, they are no
longer usable in preprocessor #if conditions; those will just have to
be open-coded or the logic rewritten into #define or runtime 'if'
conditions (but where the compiler dead-code-elimination will probably
still apply).

I tested that both gcc 10.1.1 and clang 10.0.0 produce errors for all
forms of macro mis-use.  As the errors can sometimes be cryptic, I'm
demonstrating the gcc output:

Use of MIN when MIN_CONST is needed:

In file included from /home/eblake/qemu/qemu-img.c:25:
/home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function
  249 |     ({                                                  \
      |     ^
/home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’
   92 | char array[MIN(1, 2)] = "";
      |            ^~~

Use of MIN_CONST when MIN is needed:

/home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’:
/home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be
 1225 |             i = MIN_CONST(i, n);
      |               ^

Use of MIN in the preprocessor:

In file included from /home/eblake/qemu/accel/tcg/translate-all.c:20:
/home/eblake/qemu/accel/tcg/translate-all.c: In function ‘page_check_range’:
/home/eblake/qemu/include/qemu/osdep.h:249:6: error: token "{" is not valid in preprocessor expressions
  249 |     ({                                                  \
      |      ^

Fix the resulting callsites that used #if or computed a compile-time
constant min or max to use the new macros.  cpu-defs.h is interesting,
as CPU_TLB_DYN_MAX_BITS is sometimes used as a constant and sometimes
dynamic.

It may be worth improving glib's MIN/MAX definitions to be saner, but
that is a task for another day.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200625162602.700741-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2020-06-26 09:39:39 -04:00
..
atomic_common.inc.c tcg: let plugins instrument virtual memory accesses 2019-10-28 15:12:38 +00:00
atomic_template.h trace: Remove trace_mem_build_info_no_se_[bl]e 2020-01-15 15:13:09 -10:00
cpu-exec-common.c qemu-common: Move tcg_enabled() etc. to sysemu/tcg.h 2019-06-11 20:22:09 +02:00
cpu-exec.c accel/tcg: fix race in cpu_exec_step_atomic (bug 1863025) 2020-02-28 10:58:41 -08:00
cputlb.c cputlb: destroy CPUTLB with tlb_destroy 2020-06-16 14:49:05 +01:00
Makefile.objs plugin-gen: add module for TCG-related code 2019-10-28 15:12:38 +00:00
plugin-gen.c plugin-gen: add module for TCG-related code 2019-10-28 15:12:38 +00:00
plugin-helpers.h plugin-gen: add module for TCG-related code 2019-10-28 15:12:38 +00:00
tcg-all.c qom: Drop parameter @errp of object_property_add() & friends 2020-05-15 07:07:58 +02:00
tcg-runtime-gvec.c tcg: Implement gvec support for rotate by vector 2020-06-02 08:42:37 -07:00
tcg-runtime.c tcg: Search includes from the project root source directory 2020-01-15 15:13:10 -10:00
tcg-runtime.h tcg: Implement gvec support for rotate by vector 2020-06-02 08:42:37 -07:00
trace-events accel/tcg: don't disable exec_tb trace events 2020-05-15 15:25:16 +01:00
translate-all.c osdep: Make MIN/MAX evaluate arguments only once 2020-06-26 09:39:39 -04:00
translate-all.h cputlb: Pass retaddr to tb_check_watchpoint 2019-09-25 10:56:28 -07:00
translator.c qemu_log_lock/unlock now preserves the qemu_logfile handle. 2019-12-18 20:18:02 +00:00
user-exec-stub.c hw/core: Move cpu.c, cpu.h from qom/ to hw/core/ 2019-08-21 13:24:01 +02:00
user-exec.c accel/tcg: Provide a NetBSD specific aarch64 cpu_signal_handler 2020-06-02 08:42:37 -07:00