[PATCH RFC 09/28] ARM: entry: save the syscall sp in thread_info

[Linus Walleij]

We are going to rewrite the syscall handling in C, which
means that the stack used by the call code is no longer
predicatably 8 bytes (for syscall arguments r4 and r5)
but a varying number of bytes depending on how nested the
C code is.

However the current code is just assuming it can rewind
the stack by adding 8 to sp if a syscall is interrupted by
a sigreturn call.

Solve this by storing the entry sp in the per-task
struct thread_info and use that in the sigreturn wrapper
instead. We already have the thread info available in
the SWI entry and sigreturn is probably not so common
that retrieveing a pointer to thread_info should affect
anything very much.

Storing this per-task in thread_info makes the solution
SMP robust.

Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 arch/arm/include/asm/thread_info.h | 1 +
 arch/arm/kernel/asm-offsets.c      | 1 +
 arch/arm/kernel/entry-common.S     | 8 ++++++--
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 943ffcf069d2..d8a45c5a1049 100644
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -67,6 +67,7 @@ struct thread_info {
 	__u32			cpu_domain;	/* cpu domain */
 	struct cpu_context_save	cpu_context;	/* cpu context */
 	__u32			abi_syscall;	/* ABI type and syscall nr */
+	__u32			sp_syscall;	/* SP when entering syscall */
 	unsigned long		tp_value[2];	/* TLS registers */
 	union fp_state		fpstate __attribute__((aligned(8)));
 	union vfp_state		vfpstate;
diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
index 4853875740d0..c9525cbb26b7 100644
--- a/arch/arm/kernel/asm-offsets.c
+++ b/arch/arm/kernel/asm-offsets.c
@@ -49,6 +49,7 @@ int main(void)
   DEFINE(TI_CPU_DOMAIN,		offsetof(struct thread_info, cpu_domain));
   DEFINE(TI_CPU_SAVE,		offsetof(struct thread_info, cpu_context));
   DEFINE(TI_ABI_SYSCALL,	offsetof(struct thread_info, abi_syscall));
+  DEFINE(TI_SP_SYSCALL,		offsetof(struct thread_info, sp_syscall));
   DEFINE(TI_TP_VALUE,		offsetof(struct thread_info, tp_value));
   DEFINE(TI_FPSTATE,		offsetof(struct thread_info, fpstate));
 #ifdef CONFIG_VFP
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 3cfc6d952ff9..8baab7f97f59 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -232,6 +232,8 @@ ENTRY(vector_swi)
 
 	uaccess_disable tbl
 	get_thread_info tsk
+	/* Save a per-task copy of SP for sigreturn */
+	str	sp, [tsk, #TI_SP_SYSCALL]
 
 	adr	tbl, sys_call_table		@ load syscall table pointer
 
@@ -377,13 +379,15 @@ sys_syscall:
 ENDPROC(sys_syscall)
 
 sys_sigreturn_wrapper:
-		add	r0, sp, #S_OFF
+		get_thread_info tsk
+		ldr	r0, [tsk, #TI_SP_SYSCALL]	@ read back SP
 		mov	why, #0		@ prevent syscall restart handling
 		b	sys_sigreturn
 ENDPROC(sys_sigreturn_wrapper)
 
 sys_rt_sigreturn_wrapper:
-		add	r0, sp, #S_OFF
+		get_thread_info tsk
+		ldr	r0, [tsk, #TI_SP_SYSCALL]	@ read back SP
 		mov	why, #0		@ prevent syscall restart handling
 		b	sys_rt_sigreturn
 ENDPROC(sys_rt_sigreturn_wrapper)

-- 
2.46.2