Hi Steve, et al.,

as discussed during yesterdays SFrame call I will be sending two RFC
fixup patches shortly as POC to demonstrate how this patch and
"[PATCH v11 14/15] unwind_user/sframe: Add .sframe validation option"
could benefit from introducing an internal FDE representation (e.g.
struct sframe_fde_internal) similar to the used internal FRE
representation (struct sframe_fre).

On 10/22/2025 4:43 PM, Jens Remus wrote:

+static __always_inline int __read_fde(struct sframe_section *sec,
+				      unsigned int fde_num,
+				      struct sframe_fde *fde,
+				      unsigned long *fde_start_base)

The goal would be to eliminate the passing through of fde_start_base as
well as the various computations of the effective function start address
(= *fde_start_base + fde->start_addr) throughout this module.  The
internal FDE representation could then simply convey the effective
function start address via an "unsigned long func_start_addr" field.

+{
+	unsigned long fde_addr, ip;
+
+	fde_addr = sec->fdes_start + (fde_num * sizeof(struct sframe_fde));
+	unsafe_copy_from_user(fde, (void __user *)fde_addr,
+			      sizeof(struct sframe_fde), Efault);
+
+	ip = fde_addr + fde->start_addr;
+	if (ip < sec->text_start || ip > sec->text_end)
+		return -EINVAL;
+
+	*fde_start_base = fde_addr;
+	return 0;
+
+Efault:
+	return -EFAULT;
+}
+
+static __always_inline int __find_fde(struct sframe_section *sec,
+				      unsigned long ip,
+				      struct sframe_fde *fde,
+				      unsigned long *fde_start_base)

fde_start_base would get eliminated.

+{
+	unsigned long func_addr_low = 0, func_addr_high = ULONG_MAX;
+	struct sframe_fde __user *first, *low, *high, *found = NULL;
+	int ret;
+
+	first = (void __user *)sec->fdes_start;
+	low = first;
+	high = first + sec->num_fdes - 1;
+
+	while (low <= high) {
+		struct sframe_fde __user *mid;
+		s32 func_off;
+		unsigned long func_addr;
+
+		mid = low + ((high - low) / 2);
+
+		unsafe_get_user(func_off, (s32 __user *)mid, Efault);
+		func_addr = (unsigned long)mid + func_off;
+
+		if (ip >= func_addr) {
+			if (func_addr < func_addr_low)
+				return -EFAULT;
+
+			func_addr_low = func_addr;
+
+			found = mid;
+			low = mid + 1;
+		} else {
+			if (func_addr > func_addr_high)
+				return -EFAULT;
+
+			func_addr_high = func_addr;
+
+			high = mid - 1;
+		}
+	}
+
+	if (!found)
+		return -EINVAL;
+
+	ret = __read_fde(sec, found - first, fde, fde_start_base);

fde_start_base would get eliminated.

+	if (ret)
+		return ret;
+
+	/* make sure it's not in a gap */
+	if (ip < *fde_start_base + fde->start_addr ||
+	    ip >= *fde_start_base + fde->start_addr + fde->func_size)

Would simplify to:

	if (ip < fde->func_start_addr ||
	    ip >= fde->func_start_addr + fde->func_size)

+		return -EINVAL;
+
+	return 0;
+
+Efault:
+	return -EFAULT;
+}

+static __always_inline int __find_fre(struct sframe_section *sec,
+				      struct sframe_fde *fde,
+				      unsigned long fde_start_base,

fde_start_base would get eliminated.

+				      unsigned long ip,
+				      struct unwind_user_frame *frame)
+{
+	unsigned char fde_type = SFRAME_FUNC_FDE_TYPE(fde->info);
+	struct sframe_fre *fre, *prev_fre = NULL;
+	struct sframe_fre fres[2];
+	unsigned long fre_addr;
+	bool which = false;
+	unsigned int i;
+	u32 ip_off;
+
+	ip_off = ip - (fde_start_base + fde->start_addr);

Would simplify to:

	ip_off = ip - fde->func_start_addr;

+
+	if (fde_type == SFRAME_FDE_TYPE_PCMASK)
+		ip_off %= fde->rep_size;
+
+	fre_addr = sec->fres_start + fde->fres_off;
+
+	for (i = 0; i < fde->fres_num; i++) {
+		int ret;
+
+		/*
+		 * Alternate between the two fre_addr[] entries for 'fre' and
+		 * 'prev_fre'.
+		 */
+		fre = which ? fres : fres + 1;
+		which = !which;
+
+		ret = __read_fre(sec, fde, fre_addr, fre);
+		if (ret)
+			return ret;
+
+		fre_addr += fre->size;
+
+		if (prev_fre && fre->ip_off <= prev_fre->ip_off)
+			return -EFAULT;
+
+		if (fre->ip_off > ip_off)
+			break;
+
+		prev_fre = fre;
+	}
+
+	if (!prev_fre)
+		return -EINVAL;
+	fre = prev_fre;
+
+	frame->cfa_off = fre->cfa_off;
+	frame->ra_off  = fre->ra_off;
+	frame->fp_off  = fre->fp_off;
+	frame->use_fp  = SFRAME_FRE_CFA_BASE_REG_ID(fre->info) == SFRAME_BASE_REG_FP;
+
+	return 0;
+}
+
+int sframe_find(unsigned long ip, struct unwind_user_frame *frame)
+{
+	struct mm_struct *mm = current->mm;
+	struct sframe_section *sec;
+	struct sframe_fde fde;
+	unsigned long fde_start_base;

fde_start_base would get eliminated.

+	int ret;
+
+	if (!mm)
+		return -EINVAL;
+
+	guard(srcu)(&sframe_srcu);
+
+	sec = mtree_load(&mm->sframe_mt, ip);
+	if (!sec)
+		return -EINVAL;
+
+	if (!user_read_access_begin((void __user *)sec->sframe_start,
+				    sec->sframe_end - sec->sframe_start))
+		return -EFAULT;
+
+	ret = __find_fde(sec, ip, &fde, &fde_start_base);

fde_start_base would get eliminated.

+	if (ret)
+		goto end;
+
+	ret = __find_fre(sec, &fde, fde_start_base, ip, frame);

fde_start_base would get eliminated.

+end:
+	user_read_access_end();
+	return ret;
+}

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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