[RFC PATCH v3 11/17] unwind_user: Enable archs that save RA/FP in other registers

[Jens Remus <jremus@linux.ibm.com>]

Enable unwinding of user space for architectures, such as s390, that
save the return address (RA) and/or frame pointer (FP) in other
registers.  This is only valid in the topmost frame, for instance when
in a leaf function.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Changes in RFC v3:
    - Rename UNWIND_USER_LOC_NONE to UNWIND_USER_LOC_RETAIN to better
      disambiguate from new UNWIND_USER_LOC_UNKNOWN to be introduced for
      for back chain unwinding on s390.
      Other naming options: IDENTITY, KEEP, PRESERVE, SAME, UNCHANGED.
    
    Changes in RFC v2:
    - Reword HAVE_UNWIND_USER_LOC_REG help text.
    - Rename struct unwind_user_reginfo field frame_off to offset. (Josh)
    - Move dummy unwind_user_get_reg() from asm-generic/unwind_user.h
      to linux/unwind_user.h, drop its function comment, warn once,
      return -EINVAL, and guard by !HAVE_UNWIND_USER_LOC_REG. (Josh)
    - Rename generic_sframe_set_frame_reginfo() to sframe_init_reginfo()
      and drop its function comment. (Josh)
    - Do not check FP/RA offset for zero for UNWIND_USER_LOC_STACK. (Josh)
    - Do not check for !IS_ENABLED(CONFIG_HAVE_UNWIND_USER_LOC_REG), as
      the dummy implementation of unwind_user_get_reg() returns -EINVAL.
    - Drop config option HAVE_UNWIND_USER_LOC_REG, as it is no longer of
      any value.
    - Keep checking for topmost for UNWIND_USER_LOC_REG. (Jens)
    - Explicitly preserve FP if UNWIND_USER_LOC_NONE and drop later test
      for frame->fp.loc != UNWIND_USER_LOC_NONE. (Josh)
    
    Would it make sense to rename UNWIND_USER_LOC_NONE to one of the
    following to clarify its meaning for the unwinder?
    - UNWIND_USER_LOC_UNCHANGED
    - UNWIND_USER_LOC_RETAIN
    - UNWIND_USER_LOC_PRESERVED
    - UNWIND_USER_LOC_IDENTITY

 arch/x86/include/asm/unwind_user.h       | 21 +++++++++++---
 include/asm-generic/unwind_user_sframe.h | 15 ++++++++++
 include/linux/unwind_user.h              |  9 ++++++
 include/linux/unwind_user_types.h        | 18 ++++++++++--
 kernel/unwind/sframe.c                   |  4 +--
 kernel/unwind/user.c                     | 37 +++++++++++++++++++-----
 6 files changed, 89 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index d70ffd7bbdb7..2480d86a405e 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -22,16 +22,27 @@ static inline int unwind_user_word_size(struct pt_regs *regs)
 #define ARCH_INIT_USER_FP_FRAME(ws)			\
 	.cfa_off	=  2*(ws),			\
 	.sp_off		= 0,				\
-	.ra_off		= -1*(ws),			\
-	.fp_off		= -2*(ws),			\
+	.ra		= {				\
+		.loc		= UNWIND_USER_LOC_STACK,\
+		.offset		= -1*(ws),		\
+			},				\
+	.fp		= {				\
+		.loc		= UNWIND_USER_LOC_STACK,\
+		.offset		= -2*(ws),		\
+			},				\
 	.use_fp		= true,				\
 	.outermost	= false,
 
 #define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)		\
 	.cfa_off	=  1*(ws),			\
 	.sp_off		= 0,				\
-	.ra_off		= -1*(ws),			\
-	.fp_off		= 0,				\
+	.ra		= {				\
+		.loc		= UNWIND_USER_LOC_STACK,\
+		.offset		= -1*(ws),		\
+			},				\
+	.fp		= {				\
+		.loc		= UNWIND_USER_LOC_RETAIN,\
+			},				\
 	.use_fp		= false,			\
 	.outermost	= false,
 
@@ -43,4 +54,6 @@ static inline bool unwind_user_at_function_start(struct pt_regs *regs)
 
 #endif /* CONFIG_HAVE_UNWIND_USER_FP */
 
+#include <asm-generic/unwind_user.h>
+
 #endif /* _ASM_X86_UNWIND_USER_H */
diff --git a/include/asm-generic/unwind_user_sframe.h b/include/asm-generic/unwind_user_sframe.h
index 8c9ac47bc8bd..fd71d6b1916b 100644
--- a/include/asm-generic/unwind_user_sframe.h
+++ b/include/asm-generic/unwind_user_sframe.h
@@ -2,6 +2,7 @@
 #ifndef _ASM_GENERIC_UNWIND_USER_SFRAME_H
 #define _ASM_GENERIC_UNWIND_USER_SFRAME_H
 
+#include <linux/unwind_user_types.h>
 #include <linux/types.h>
 
 #ifndef SFRAME_SP_OFFSET
@@ -9,4 +10,18 @@
 #define SFRAME_SP_OFFSET 0
 #endif
 
+#ifndef sframe_init_reginfo
+static inline void
+sframe_init_reginfo(struct unwind_user_reginfo *reginfo, s32 offset)
+{
+	if (offset) {
+		reginfo->loc = UNWIND_USER_LOC_STACK;
+		reginfo->offset = offset;
+	} else {
+		reginfo->loc = UNWIND_USER_LOC_RETAIN;
+	}
+}
+#define sframe_init_reginfo sframe_init_reginfo
+#endif
+
 #endif /* _ASM_GENERIC_UNWIND_USER_SFRAME_H */
diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
index bc2edae39955..61fd5c05d0f0 100644
--- a/include/linux/unwind_user.h
+++ b/include/linux/unwind_user.h
@@ -32,6 +32,15 @@ static inline int unwind_user_get_ra_reg(unsigned long *val)
 #define unwind_user_get_ra_reg unwind_user_get_ra_reg
 #endif
 
+#ifndef unwind_user_get_reg
+static inline int unwind_user_get_reg(unsigned long *val, int regnum)
+{
+	WARN_ON_ONCE(1);
+	return -EINVAL;
+}
+#define unwind_user_get_reg unwind_user_get_reg
+#endif
+
 int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries);
 
 #endif /* _LINUX_UNWIND_USER_H */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 4656aa08a7db..4f78999a0750 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -27,11 +27,25 @@ struct unwind_stacktrace {
 	unsigned long	*entries;
 };
 
+enum unwind_user_loc {
+	UNWIND_USER_LOC_RETAIN,
+	UNWIND_USER_LOC_STACK,
+	UNWIND_USER_LOC_REG,
+};
+
+struct unwind_user_reginfo {
+	enum unwind_user_loc loc;
+	union {
+		s32 offset;
+		int regnum;
+	};
+};
+
 struct unwind_user_frame {
 	s32 cfa_off;
 	s32 sp_off;
-	s32 ra_off;
-	s32 fp_off;
+	struct unwind_user_reginfo ra;
+	struct unwind_user_reginfo fp;
 	bool use_fp;
 	bool outermost;
 };
diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index 38b3577f5253..45cd7380ac38 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -307,8 +307,8 @@ static __always_inline int __find_fre(struct sframe_section *sec,
 
 	frame->cfa_off = fre->cfa_off;
 	frame->sp_off  = SFRAME_SP_OFFSET;
-	frame->ra_off  = fre->ra_off;
-	frame->fp_off  = fre->fp_off;
+	sframe_init_reginfo(&frame->ra, fre->ra_off);
+	sframe_init_reginfo(&frame->fp, fre->fp_off);
 	frame->use_fp  = SFRAME_FRE_CFA_BASE_REG_ID(fre->info) == SFRAME_BASE_REG_FP;
 	frame->outermost = fre->ra_undefined;
 
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 58e1549cd9f4..45f82ed28fcb 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -62,22 +62,45 @@ static int unwind_user_next_common(struct unwind_user_state *state,
 		return -EINVAL;
 
 	/* Get the Return Address (RA) */
-	if (frame->ra_off) {
-		if (get_user_word(&ra, cfa, frame->ra_off, state->ws))
-			return -EINVAL;
-	} else {
+	switch (frame->ra.loc) {
+	case UNWIND_USER_LOC_RETAIN:
 		if (!state->topmost || unwind_user_get_ra_reg(&ra))
 			return -EINVAL;
+		break;
+	case UNWIND_USER_LOC_STACK:
+		if (get_user_word(&ra, cfa, frame->ra.offset, state->ws))
+			return -EINVAL;
+		break;
+	case UNWIND_USER_LOC_REG:
+		if (!state->topmost || unwind_user_get_reg(&ra, frame->ra.regnum))
+			return -EINVAL;
+		break;
+	default:
+		WARN_ON_ONCE(1);
+		return -EINVAL;
 	}
 
 	/* Get the Frame Pointer (FP) */
-	if (frame->fp_off && get_user_word(&fp, cfa, frame->fp_off, state->ws))
+	switch (frame->fp.loc) {
+	case UNWIND_USER_LOC_RETAIN:
+		fp = state->fp;
+		break;
+	case UNWIND_USER_LOC_STACK:
+		if (get_user_word(&fp, cfa, frame->fp.offset, state->ws))
+			return -EINVAL;
+		break;
+	case UNWIND_USER_LOC_REG:
+		if (!state->topmost || unwind_user_get_reg(&fp, frame->fp.regnum))
+			return -EINVAL;
+		break;
+	default:
+		WARN_ON_ONCE(1);
 		return -EINVAL;
+	}
 
 	state->ip = ra;
 	state->sp = sp;
-	if (frame->fp_off)
-		state->fp = fp;
+	state->fp = fp;
 	state->topmost = false;
 	return 0;
 }
-- 
2.51.0