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Peter Anvin" Subject: [PATCH -tip 1/1] x86_32/segment: Always return correctly zero-extended values from savesegment_*() Date: Tue, 14 Apr 2026 09:44:42 +0200 Message-ID: <20260414074613.77330-2-ubizjak@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260414074613.77330-1-ubizjak@gmail.com> References: <20260414074613.77330-1-ubizjak@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On 32-bit x86, reading segment registers into a 32-bit type can expose undefined upper bits on older processors (e.g. Intel Quark X1000, Pentium, and earlier), where bits 31:16 are not defined. Introduce __seg_return_t as an intermediate type for __savesegment_*(): u16 on CONFIG_X86_32 and unsigned long otherwise. As a result, __savesegment_*() now returns correctly zero-extended value in all cases, avoiding propagation of undefined high bits on 32-bit systems while preserving existing behavior on 64-bit. Signed-off-by: Uros Bizjak Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" --- arch/x86/include/asm/segment.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h index dbd90fede5e7..070ed87063e0 100644 --- a/arch/x86/include/asm/segment.h +++ b/arch/x86/include/asm/segment.h @@ -341,13 +341,23 @@ static inline void __loadsegment_fs(u16 value) #define loadsegment(seg, val) __loadsegment_##seg(val) +#ifdef CONFIG_X86_32 +/* + * Bits 31:16 are undefined for Intel Quark X1000 processors, + * Pentium, and earlier processors. + */ +typedef u16 __seg_return_t; +#else +typedef unsigned long __seg_return_t; +#endif + /* * Save a segment register away: */ #define SAVE_SEGMENT(seg) \ static inline unsigned long __savesegment_##seg(void) \ { \ - unsigned long v; \ + __seg_return_t v; \ asm volatile("movl %%" #seg ",%k0" : "=r" (v)); \ return v; \ } -- 2.53.0