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Re: [PATCH] hw/arm_sysctl: fix extraxting 31th bit of val


From: Philippe Mathieu-Daudé
Subject: Re: [PATCH] hw/arm_sysctl: fix extraxting 31th bit of val
Date: Fri, 20 Dec 2024 11:44:57 +0100
User-agent: Mozilla Thunderbird

On 20/12/24 11:33, Anastasia Belova wrote:
1 << 31 is casted to uint64_t while bitwise and with val.
So this value may become 0xffffffff80000000 but only
31th "start" bit is required.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
---
  hw/misc/arm_sysctl.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/misc/arm_sysctl.c b/hw/misc/arm_sysctl.c
index 69e379fa10..ce8be44b6e 100644
--- a/hw/misc/arm_sysctl.c
+++ b/hw/misc/arm_sysctl.c
@@ -520,7 +520,7 @@ static void arm_sysctl_write(void *opaque, hwaddr offset,
           * as zero.
           */
          s->sys_cfgctrl = val & ~((3 << 18) | (1 << 31));
-        if (val & (1 << 31)) {
+        if (val & (1ULL << 31)) {

Alternatively clearer using the bitfield extract() API ...:

           if (extract64(val, 31, 1)) {

              /* Start bit set -- actually do something */
              unsigned int dcc = extract32(s->sys_cfgctrl, 26, 4);
              unsigned int function = extract32(s->sys_cfgctrl, 20, 6);

... which we already use in this file   ^^^^^^^.



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