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Code patching for the dynamically enabled shadow call stack comes down to finding PACIASP and AUTIASP instructions -which behave as NOPs on cores that do not implement pointer authentication- and converting them into shadow call stack pushes and pops, respectively. Due to past bad experiences with the highly complex and overengineered DWARF standard that describes the unwind metadata that we are using to locate these instructions, let's make this patching logic a little bit more robust so that any issues with the unwind metadata detected at boot time can de dealt with gracefully. The DWARF annotations that are used for this are emitted at function granularity, and due to the fact that the instructions we are patching will simply behave as NOPs if left unpatched, we can abort on errors as long as we don't leave any functions in a half-patched state. So do a dry run of each FDE frame (covering a single function) before performing the actual patching, and give up if the DWARF metadata cannot be understood. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> Acked-by: Will Deacon <will@kernel.org> Reviewed-by: Sami Tolvanen <samitolvanen@google.com> Link: https://lore.kernel.org/r/20221213142849.1629026-1-ardb@kernel.org Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
263 lines
6.6 KiB
C
263 lines
6.6 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Copyright (C) 2022 - Google LLC
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* Author: Ard Biesheuvel <ardb@google.com>
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*/
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#include <linux/bug.h>
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#include <linux/errno.h>
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#include <linux/init.h>
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#include <linux/linkage.h>
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#include <linux/printk.h>
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#include <linux/types.h>
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#include <asm/cacheflush.h>
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#include <asm/scs.h>
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//
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// This minimal DWARF CFI parser is partially based on the code in
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// arch/arc/kernel/unwind.c, and on the document below:
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// https://refspecs.linuxbase.org/LSB_4.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html
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//
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#define DW_CFA_nop 0x00
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#define DW_CFA_set_loc 0x01
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#define DW_CFA_advance_loc1 0x02
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#define DW_CFA_advance_loc2 0x03
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#define DW_CFA_advance_loc4 0x04
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#define DW_CFA_offset_extended 0x05
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#define DW_CFA_restore_extended 0x06
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#define DW_CFA_undefined 0x07
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#define DW_CFA_same_value 0x08
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#define DW_CFA_register 0x09
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#define DW_CFA_remember_state 0x0a
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#define DW_CFA_restore_state 0x0b
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#define DW_CFA_def_cfa 0x0c
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#define DW_CFA_def_cfa_register 0x0d
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#define DW_CFA_def_cfa_offset 0x0e
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#define DW_CFA_def_cfa_expression 0x0f
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#define DW_CFA_expression 0x10
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#define DW_CFA_offset_extended_sf 0x11
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#define DW_CFA_def_cfa_sf 0x12
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#define DW_CFA_def_cfa_offset_sf 0x13
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#define DW_CFA_val_offset 0x14
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#define DW_CFA_val_offset_sf 0x15
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#define DW_CFA_val_expression 0x16
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#define DW_CFA_lo_user 0x1c
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#define DW_CFA_negate_ra_state 0x2d
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#define DW_CFA_GNU_args_size 0x2e
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#define DW_CFA_GNU_negative_offset_extended 0x2f
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#define DW_CFA_hi_user 0x3f
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extern const u8 __eh_frame_start[], __eh_frame_end[];
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enum {
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PACIASP = 0xd503233f,
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AUTIASP = 0xd50323bf,
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SCS_PUSH = 0xf800865e,
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SCS_POP = 0xf85f8e5e,
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};
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static void __always_inline scs_patch_loc(u64 loc)
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{
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u32 insn = le32_to_cpup((void *)loc);
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switch (insn) {
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case PACIASP:
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*(u32 *)loc = cpu_to_le32(SCS_PUSH);
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break;
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case AUTIASP:
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*(u32 *)loc = cpu_to_le32(SCS_POP);
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break;
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default:
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/*
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* While the DW_CFA_negate_ra_state directive is guaranteed to
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* appear right after a PACIASP/AUTIASP instruction, it may
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* also appear after a DW_CFA_restore_state directive that
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* restores a state that is only partially accurate, and is
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* followed by DW_CFA_negate_ra_state directive to toggle the
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* PAC bit again. So we permit other instructions here, and ignore
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* them.
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*/
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return;
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}
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dcache_clean_pou(loc, loc + sizeof(u32));
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}
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/*
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* Skip one uleb128/sleb128 encoded quantity from the opcode stream. All bytes
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* except the last one have bit #7 set.
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*/
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static int __always_inline skip_xleb128(const u8 **opcode, int size)
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{
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u8 c;
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do {
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c = *(*opcode)++;
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size--;
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} while (c & BIT(7));
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return size;
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}
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struct eh_frame {
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/*
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* The size of this frame if 0 < size < U32_MAX, 0 terminates the list.
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*/
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u32 size;
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/*
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* The first frame is a Common Information Entry (CIE) frame, followed
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* by one or more Frame Description Entry (FDE) frames. In the former
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* case, this field is 0, otherwise it is the negated offset relative
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* to the associated CIE frame.
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*/
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u32 cie_id_or_pointer;
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union {
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struct { // CIE
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u8 version;
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u8 augmentation_string[];
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};
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struct { // FDE
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s32 initial_loc;
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s32 range;
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u8 opcodes[];
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};
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};
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};
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static int noinstr scs_handle_fde_frame(const struct eh_frame *frame,
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bool fde_has_augmentation_data,
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int code_alignment_factor,
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bool dry_run)
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{
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int size = frame->size - offsetof(struct eh_frame, opcodes) + 4;
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u64 loc = (u64)offset_to_ptr(&frame->initial_loc);
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const u8 *opcode = frame->opcodes;
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if (fde_has_augmentation_data) {
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int l;
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// assume single byte uleb128_t
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if (WARN_ON(*opcode & BIT(7)))
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return -ENOEXEC;
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l = *opcode++;
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opcode += l;
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size -= l + 1;
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}
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/*
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* Starting from 'loc', apply the CFA opcodes that advance the location
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* pointer, and identify the locations of the PAC instructions.
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*/
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while (size-- > 0) {
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switch (*opcode++) {
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case DW_CFA_nop:
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case DW_CFA_remember_state:
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case DW_CFA_restore_state:
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break;
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case DW_CFA_advance_loc1:
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loc += *opcode++ * code_alignment_factor;
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size--;
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break;
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case DW_CFA_advance_loc2:
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loc += *opcode++ * code_alignment_factor;
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loc += (*opcode++ << 8) * code_alignment_factor;
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size -= 2;
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break;
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case DW_CFA_def_cfa:
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case DW_CFA_offset_extended:
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size = skip_xleb128(&opcode, size);
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fallthrough;
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case DW_CFA_def_cfa_offset:
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case DW_CFA_def_cfa_offset_sf:
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case DW_CFA_def_cfa_register:
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case DW_CFA_same_value:
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case DW_CFA_restore_extended:
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case 0x80 ... 0xbf:
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size = skip_xleb128(&opcode, size);
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break;
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case DW_CFA_negate_ra_state:
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if (!dry_run)
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scs_patch_loc(loc - 4);
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break;
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case 0x40 ... 0x7f:
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// advance loc
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loc += (opcode[-1] & 0x3f) * code_alignment_factor;
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break;
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case 0xc0 ... 0xff:
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break;
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default:
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pr_err("unhandled opcode: %02x in FDE frame %lx\n", opcode[-1], (uintptr_t)frame);
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return -ENOEXEC;
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}
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}
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return 0;
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}
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int noinstr scs_patch(const u8 eh_frame[], int size)
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{
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const u8 *p = eh_frame;
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while (size > 4) {
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const struct eh_frame *frame = (const void *)p;
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bool fde_has_augmentation_data = true;
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int code_alignment_factor = 1;
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int ret;
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if (frame->size == 0 ||
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frame->size == U32_MAX ||
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frame->size > size)
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break;
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if (frame->cie_id_or_pointer == 0) {
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const u8 *p = frame->augmentation_string;
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/* a 'z' in the augmentation string must come first */
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fde_has_augmentation_data = *p == 'z';
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/*
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* The code alignment factor is a uleb128 encoded field
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* but given that the only sensible values are 1 or 4,
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* there is no point in decoding the whole thing.
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*/
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p += strlen(p) + 1;
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if (!WARN_ON(*p & BIT(7)))
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code_alignment_factor = *p;
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} else {
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ret = scs_handle_fde_frame(frame,
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fde_has_augmentation_data,
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code_alignment_factor,
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true);
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if (ret)
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return ret;
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scs_handle_fde_frame(frame, fde_has_augmentation_data,
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code_alignment_factor, false);
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}
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p += sizeof(frame->size) + frame->size;
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size -= sizeof(frame->size) + frame->size;
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}
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return 0;
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}
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asmlinkage void __init scs_patch_vmlinux(void)
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{
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if (!should_patch_pac_into_scs())
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return;
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WARN_ON(scs_patch(__eh_frame_start, __eh_frame_end - __eh_frame_start));
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icache_inval_all_pou();
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isb();
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}
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