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// SPDX-License-Identifier: GPL-2.0-or-later
#include <string.h>
#include <objtool/special.h>
#include <objtool/builtin.h>
#define X86_FEATURE_POPCNT (4 * 32 + 23)
#define X86_FEATURE_SMAP (9 * 32 + 20)
void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
{
switch (feature) {
case X86_FEATURE_SMAP:
/*
* If UACCESS validation is enabled; force that alternative;
* otherwise force it the other way.
*
* What we want to avoid is having both the original and the
* alternative code flow at the same time, in that case we can
* find paths that see the STAC but take the NOP instead of
* CLAC and the other way around.
*/
if (uaccess)
alt->skip_orig = true;
else
alt->skip_alt = true;
break;
case X86_FEATURE_POPCNT:
/*
* It has been requested that we don't validate the !POPCNT
* feature path which is a "very very small percentage of
* machines".
*/
alt->skip_orig = true;
break;
default:
break;
}
}
bool arch_support_alt_relocation(struct special_alt *special_alt,
struct instruction *insn,
struct reloc *reloc)
{
/*
* The x86 alternatives code adjusts the offsets only when it
* encounters a branch instruction at the very beginning of the
* replacement group.
*/
return insn->offset == special_alt->new_off &&
(insn->type == INSN_CALL || is_jump(insn));
}
/*
* There are 3 basic jump table patterns:
*
* 1. jmpq *[rodata addr](,%reg,8)
*
* This is the most common case by far. It jumps to an address in a simple
* jump table which is stored in .rodata.
*
* 2. jmpq *[rodata addr](%rip)
*
* This is caused by a rare GCC quirk, currently only seen in three driver
* functions in the kernel, only with certain obscure non-distro configs.
*
* As part of an optimization, GCC makes a copy of an existing switch jump
* table, modifies it, and then hard-codes the jump (albeit with an indirect
* jump) to use a single entry in the table. The rest of the jump table and
* some of its jump targets remain as dead code.
*
* In such a case we can just crudely ignore all unreachable instruction
* warnings for the entire object file. Ideally we would just ignore them
* for the function, but that would require redesigning the code quite a
* bit. And honestly that's just not worth doing: unreachable instruction
* warnings are of questionable value anyway, and this is such a rare issue.
*
* 3. mov [rodata addr],%reg1
* ... some instructions ...
* jmpq *(%reg1,%reg2,8)
*
* This is a fairly uncommon pattern which is new for GCC 6. As of this
* writing, there are 11 occurrences of it in the allmodconfig kernel.
*
* As of GCC 7 there are quite a few more of these and the 'in between' code
* is significant. Esp. with KASAN enabled some of the code between the mov
* and jmpq uses .rodata itself, which can confuse things.
*
* TODO: Once we have DWARF CFI and smarter instruction decoding logic,
* ensure the same register is used in the mov and jump instructions.
*
* NOTE: RETPOLINE made it harder still to decode dynamic jumps.
*/
struct reloc *arch_find_switch_table(struct objtool_file *file,
struct instruction *insn)
{
struct reloc *text_reloc, *rodata_reloc;
struct section *table_sec;
unsigned long table_offset;
/* look for a relocation which references .rodata */
text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
insn->offset, insn->len);
if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
!text_reloc->sym->sec->rodata)
return NULL;
table_offset = text_reloc->addend;
table_sec = text_reloc->sym->sec;
if (text_reloc->type == R_X86_64_PC32)
table_offset += 4;
/*
* Make sure the .rodata address isn't associated with a
* symbol. GCC jump tables are anonymous data.
*
* Also support C jump tables which are in the same format as
* switch jump tables. For objtool to recognize them, they
* need to be placed in the C_JUMP_TABLE_SECTION section. They
* have symbols associated with them.
*/
if (find_symbol_containing(table_sec, table_offset) &&
strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
return NULL;
/*
* Each table entry has a rela associated with it. The rela
* should reference text in the same function as the original
* instruction.
*/
rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
if (!rodata_reloc)
return NULL;
/*
* Use of RIP-relative switch jumps is quite rare, and
* indicates a rare GCC quirk/bug which can leave dead
* code behind.
*/
if (text_reloc->type == R_X86_64_PC32)
file->ignore_unreachables = true;
return rodata_reloc;
}
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