Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat
On Fri, Apr 16, 2021 at 03:55:32PM +0800, He Zhe wrote:
Add sign extension handling in syscall_get_return_value so that it
can
handle 32-bit compatible case and can be used by for example audit, just
like what syscall_get_error does.
If a compat syscall can ever legitimately return a non-error value with
bit 31 set, and this sign-extends it, is that ever going to reach
userspace as a 64-bit value?
IIUC things like mmap() can return pointers above 2GiB for a compat
task, so I'm a bit uneasy that we'd handle those wrong. I can't see a
way of preventing that unless we keep the upper 32 bits for errors.
Mark.
Signed-off-by: He Zhe <zhe.he(a)windriver.com>
---
arch/arm64/include/asm/syscall.h | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index cfc0672013f6..cd7a22787aeb 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -44,7 +44,12 @@ static inline long syscall_get_error(struct task_struct *task,
static inline long syscall_get_return_value(struct task_struct *task,
struct pt_regs *regs)
{
- return regs->regs[0];
+ long val = regs->regs[0];
+
+ if (is_compat_thread(task_thread_info(task)))
+ val = sign_extend64(val, 31);
+
+ return val;
}
static inline void syscall_set_return_value(struct task_struct *task,
--
2.17.1