Re: [patch V3 15/20] x86/fpu/signal: Change return type of copy_fpregs_to_sigframe() helpers to boolean

From: Anders Roxell
Date: Tue Sep 21 2021 - 06:58:50 EST


On Wed, 8 Sept 2021 at 15:30, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Now that copy_fpregs_to_sigframe() returns boolean the individual return
> codes in the related helper functions do not make sense anymore. Change
> them to return boolean success/fail.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

When I build and boot (qemu_x86_64) a defconfig kernel on from todays
next tag next-20210921 I see the following segmentation fault

2021-09-21T10:11:45 <6>[ 1.622922] mount (89) used greatest stack
depth: 14384 bytes left
2021-09-21T10:11:45 <6>[ 1.664760] EXT4-fs (sda): re-mounted. Opts:
(null). Quota mode: none.
2021-09-21T10:11:45 <6>[ 1.691041] mkdir (92) used greatest stack
depth: 14312 bytes left
2021-09-21T10:11:45 <6>[ 1.713201] mount (93) used greatest stack
depth: 13720 bytes left
2021-09-21T10:11:46 Starting syslogd: /etc/init.d/rcS: line 12: 101
Segmentation fault $i start


I did a bisection and found this as the faulty patch [1]. When I
revert this patch I can't see the issue.

We noticed that function 'save_xstate_epilog()' changes the polarity
of its return code for one of the return statements, and for its only
caller. but not for the other return statement.

I tried this patch and I couldn't see the segmentation fault.

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 445c57c9c539..61eeebc04427 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -104,7 +104,7 @@ static inline int save_xstate_epilog(void __user
*buf, int ia32_frame)
err = __copy_to_user(&x->i387.sw_reserved, sw_bytes, sizeof(*sw_bytes));

if (!use_xsave())
- return err;
+ return !err;

err |= __put_user(FP_XSTATE_MAGIC2,
(__u32 __user *)(buf + fpu_user_xstate_size));



Cheers,
Anders
[1] http://ix.io/3zxf

> ---
> arch/x86/kernel/fpu/signal.c | 17 +++++++++--------
> 1 file changed, 9 insertions(+), 8 deletions(-)
>
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -65,7 +65,7 @@ static inline int check_xstate_in_sigfra
> /*
> * Signal frame handlers.
> */
> -static inline int save_fsave_header(struct task_struct *tsk, void __user *buf)
> +static inline bool save_fsave_header(struct task_struct *tsk, void __user *buf)
> {
> if (use_fxsr()) {
> struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
> @@ -82,18 +82,19 @@ static inline int save_fsave_header(stru
> if (__copy_to_user(buf, &env, sizeof(env)) ||
> __put_user(xsave->i387.swd, &fp->status) ||
> __put_user(X86_FXSR_MAGIC, &fp->magic))
> - return -1;
> + return false;
> } else {
> struct fregs_state __user *fp = buf;
> u32 swd;
> +
> if (__get_user(swd, &fp->swd) || __put_user(swd, &fp->status))
> - return -1;
> + return false;
> }
>
> - return 0;
> + return true;
> }
>
> -static inline int save_xstate_epilog(void __user *buf, int ia32_frame)
> +static inline bool save_xstate_epilog(void __user *buf, int ia32_frame)
> {
> struct xregs_state __user *x = buf;
> struct _fpx_sw_bytes *sw_bytes;
> @@ -131,7 +132,7 @@ static inline int save_xstate_epilog(voi
>
> err |= __put_user(xfeatures, (__u32 __user *)&x->header.xfeatures);
>
> - return err;
> + return !err;
> }
>
> static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
> @@ -219,10 +220,10 @@ bool copy_fpstate_to_sigframe(void __use
> }
>
> /* Save the fsave header for the 32-bit frames. */
> - if ((ia32_fxstate || !use_fxsr()) && save_fsave_header(tsk, buf))
> + if ((ia32_fxstate || !use_fxsr()) && !save_fsave_header(tsk, buf))
> return false;
>
> - if (use_fxsr() && save_xstate_epilog(buf_fx, ia32_fxstate))
> + if (use_fxsr() && !save_xstate_epilog(buf_fx, ia32_fxstate))
> return false;
>
> return true;
>