Re: [PATCH v11 06/15] unwind_user/sframe: Add support for reading .sframe contents

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

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:

> diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c

> +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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