2023-12-19 15:12:59

by Vegard Nossum

Subject: [PATCH 5/5] x86: add DB flag to 32-bit percpu GDT entry

The D/B size flag for the 32-bit percpu GDT entry was not set.

The Intel manual (vol 3, section 3.4.5) only specifies the meaning of
this flag for three cases:

1) code segments used for %cs -- doesn't apply here
2) stack segments used for %ss -- doesn't apply
3) expand-down data segments -- but we don't have the expand-down flag
set, so it also doesn't apply here

The flag likely doesn't do anything here, although the manual does also
say: "This flag should always be set to 1 for 32-bit code and data
segments [...]" so we should probably do it anyway.

Link: https://lore.kernel.org/all/CAHk-=wib5XLebuEra7y2YH96wxdk=8vJnA8XoVq0FExpzVvN=Q@mail.gmail.com/
Signed-off-by: Vegard Nossum <[email protected]>
---
arch/x86/kernel/setup_percpu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index f2583de97a64..b30d6e180df7 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -106,7 +106,7 @@ void __init pcpu_populate_pte(unsigned long addr)
static inline void setup_percpu_segment(int cpu)
{
#ifdef CONFIG_X86_32
- struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32 & ~_DESC_DB,
+ struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32,
per_cpu_offset(cpu), 0xFFFFF);

write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);
--
2.34.1

2023-12-20 10:05:57

by tip-bot2 for Dhananjay Ugwekar

Subject: [tip: x86/asm] x86/asm: Add DB flag to 32-bit percpu GDT entry

The following commit has been merged into the x86/asm branch of tip:

Commit-ID: bc90aefa99f74452d549d503a3f1cbf3adc9c6bb
Gitweb: https://git.kernel.org/tip/bc90aefa99f74452d549d503a3f1cbf3adc9c6bb
Author: Vegard Nossum <[email protected]>
AuthorDate: Tue, 19 Dec 2023 16:12:00 +01:00
Committer: Ingo Molnar <[email protected]>
CommitterDate: Wed, 20 Dec 2023 10:57:51 +01:00

x86/asm: Add DB flag to 32-bit percpu GDT entry

The D/B size flag for the 32-bit percpu GDT entry was not set.

The Intel manual (vol 3, section 3.4.5) only specifies the meaning of
this flag for three cases:

1) code segments used for %cs -- doesn't apply here

2) stack segments used for %ss -- doesn't apply

3) expand-down data segments -- but we don't have the expand-down flag
set, so it also doesn't apply here

The flag likely doesn't do anything here, although the manual does also
say: "This flag should always be set to 1 for 32-bit code and data
segments [...]" so we should probably do it anyway.

Signed-off-by: Vegard Nossum <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/kernel/setup_percpu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index f2583de..b30d6e1 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -106,7 +106,7 @@ void __init pcpu_populate_pte(unsigned long addr)
static inline void setup_percpu_segment(int cpu)
{
#ifdef CONFIG_X86_32
- struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32 & ~_DESC_DB,
+ struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32,
per_cpu_offset(cpu), 0xFFFFF);

write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);