[PATCH v14 17/19] unwind_user/sframe: Separate reading of FRE from reading of FRE data words

Jens Remus
2026-05-05T12:18:23Z

__find_fre() performs linear search for a matching SFrame FRE for a
given IP. For that purpose it uses __read_fre(), which reads the whole
FRE. That is the variable-size FRE structure as well as the trailing
variable-length array of variable-size data words. For the search logic
to skip over the FRE it would be sufficient to read the variable-size
FRE structure only, which includes the count and size of data words.

Add fields to struct sframe_fre_internal to store the FRE data word's
address, count, and size. Change __read_fre() to read the variable-
size FRE structure only and populate those new fields. Change
__read_fre_datawords() to use those new fields. Change __find_fre()
to use __read_fre_datawords() to read the FRE data words only after a
matching FRE has been found. Introduce safe_read_fre_datawords() and
use it in sframe_validate_section() to validate that the FRE data words.

Reviewed-by: Indu Bhagat <ibhagatgnu@gmail.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
Changes in v14:
- Adjust to rename of SFRAME_FDE_TYPE_* and
__read_default_fre_datawords().
- Update function name in debug message.

kernel/unwind/sframe.c | 91 +++++++++++++++++++++++++++---------------
1 file changed, 58 insertions(+), 33 deletions(-)

diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index 2ba2c8b385f9..98346412078b 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -39,6 +39,9 @@ struct sframe_fre_internal {
u32 fp_ctl;
s32 fp_off;
u8 info;
+ unsigned long dw_addr;
+ unsigned char dw_count;
+ unsigned char dw_size;
};

DEFINE_STATIC_SRCU(sframe_srcu);
@@ -196,11 +199,11 @@ static __always_inline int __find_fde(struct sframe_section *sec,
static __always_inline int
__read_default_fre_datawords(struct sframe_section *sec,
struct sframe_fde_internal *fde,
- unsigned long cur,
- unsigned char dataword_count,
- unsigned char dataword_size,
struct sframe_fre_internal *fre)
{
+ unsigned char dataword_count = fre->dw_count;
+ unsigned char dataword_size = fre->dw_size;
+ unsigned long cur = fre->dw_addr;
s32 cfa_off, ra_off, fp_off;
unsigned int cfa_regnum;

@@ -242,11 +245,11 @@ __read_default_fre_datawords(struct sframe_section *sec,
static __always_inline int
__read_flex_fde_fre_datawords(struct sframe_section *sec,
struct sframe_fde_internal *fde,
- unsigned long cur,
- unsigned char dataword_count,
- unsigned char dataword_size,
struct sframe_fre_internal *fre)
{
+ unsigned char dataword_count = fre->dw_count;
+ unsigned char dataword_size = fre->dw_size;
+ unsigned long cur = fre->dw_addr;
u32 cfa_ctl, ra_ctl, fp_ctl;
s32 cfa_off, ra_off, fp_off;

@@ -303,24 +306,28 @@ __read_flex_fde_fre_datawords(struct sframe_section *sec,
static __always_inline int
__read_fre_datawords(struct sframe_section *sec,
struct sframe_fde_internal *fde,
- unsigned long cur,
- unsigned char dataword_count,
- unsigned char dataword_size,
struct sframe_fre_internal *fre)
{
unsigned char fde_type = SFRAME_V3_FDE_TYPE(fde->info2);
+ unsigned char dataword_count = fre->dw_count;
+
+ if (!dataword_count) {
+ /* A FRE without data words indicates an outermost frame. */
+ fre->cfa_ctl = 0;
+ fre->cfa_off = 0;
+ fre->ra_ctl = 0;
+ fre->ra_off = 0;
+ fre->fp_ctl = 0;
+ fre->fp_off = 0;
+
+ return 0;
+ }

switch (fde_type) {
case SFRAME_FDE_TYPE_DEFAULT:
- return __read_default_fre_datawords(sec, fde, cur,
- dataword_count,
- dataword_size,
- fre);
+ return __read_default_fre_datawords(sec, fde, fre);
case SFRAME_FDE_TYPE_FLEX:
- return __read_flex_fde_fre_datawords(sec, fde, cur,
- dataword_count,
- dataword_size,
- fre);
+ return __read_flex_fde_fre_datawords(sec, fde, fre);
default:
return -EFAULT;
}
@@ -362,23 +369,11 @@ static __always_inline int __read_fre(struct sframe_section *sec,
fre->size = addr_size + 1 + (dataword_count * dataword_size);
fre->ip_off = ip_off;
fre->info = info;
+ fre->dw_addr = cur;
+ fre->dw_count = dataword_count;
+ fre->dw_size = dataword_size;

- if (!dataword_count) {
- /*
- * A FRE without data words indicates RA undefined /
- * outermost frame.
- */
- fre->cfa_ctl = 0;
- fre->cfa_off = 0;
- fre->ra_ctl = 0;
- fre->ra_off = 0;
- fre->fp_ctl = 0;
- fre->fp_off = 0;
-
- return 0;
- }
-
- return __read_fre_datawords(sec, fde, cur, dataword_count, dataword_size, fre);
+ return 0;

Efault:
return -EFAULT;
@@ -455,6 +450,7 @@ static __always_inline int __find_fre(struct sframe_section *sec,
bool which = false;
unsigned int i;
u32 ip_off;
+ int ret;

ip_off = ip - fde->func_addr;

@@ -492,6 +488,10 @@ static __always_inline int __find_fre(struct sframe_section *sec,
return -EINVAL;
fre = prev_fre;

+ ret = __read_fre_datawords(sec, fde, fre);
+ if (ret)
+ return ret;
+
if (sframe_init_cfa_rule_data(&frame->cfa, fre->cfa_ctl, fre->cfa_off))
return -EINVAL;
sframe_init_rule_data(&frame->ra, fre->ra_ctl, fre->ra_off);
@@ -567,6 +567,20 @@ static int safe_read_fre(struct sframe_section *sec,
return ret;
}

+static int safe_read_fre_datawords(struct sframe_section *sec,
+ struct sframe_fde_internal *fde,
+ struct sframe_fre_internal *fre)
+{
+ int ret;
+
+ if (!user_read_access_begin((void __user *)sec->sframe_start,
+ sec->sframe_end - sec->sframe_start))
+ return -EFAULT;
+ ret = __read_fre_datawords(sec, fde, fre);
+ user_read_access_end();
+ return ret;
+}
+
static int sframe_validate_section(struct sframe_section *sec)
{
unsigned long prev_ip = 0;
@@ -612,6 +626,17 @@ static int sframe_validate_section(struct sframe_section *sec)
fde.rep_size);
return ret;
}
+ ret = safe_read_fre_datawords(sec, &fde, fre);
+ if (ret) {
+ dbg_sec("FDE %u: safe_read_fre_datawords(%u) failed\n", i, j);
+ dbg_sec("FDE: func_addr:%#lx func_size:%#x fda_off:%#x fres_off:%#x fres_num:%d info:%u info2:%u rep_size:%u\n",
+ fde.func_addr, fde.func_size,
+ fde.fda_off,
+ fde.fres_off, fde.fres_num,
+ fde.info, fde.info2,
+ fde.rep_size);
+ return ret;
+ }

fre_addr += fre->size;

--
2.51.0