linux/tools/objtool/orc_gen.c
Josh Poimboeuf c9c324dc22 objtool: Support stack layout changes in alternatives
The ORC unwinder showed a warning [1] which revealed the stack layout
didn't match what was expected.  The problem was that paravirt patching
had replaced "CALL *pv_ops.irq.save_fl" with "PUSHF;POP".  That changed
the stack layout between the PUSHF and the POP, so unwinding from an
interrupt which occurred between those two instructions would fail.

Part of the agreed upon solution was to rework the custom paravirt
patching code to use alternatives instead, since objtool already knows
how to read alternatives (and converging runtime patching infrastructure
is always a good thing anyway).  But the main problem still remains,
which is that runtime patching can change the stack layout.

Making stack layout changes in alternatives was disallowed with commit
7117f16bf4 ("objtool: Fix ORC vs alternatives"), but now that paravirt
is going to be doing it, it needs to be supported.

One way to do so would be to modify the ORC table when the code gets
patched.  But ORC is simple -- a good thing! -- and it's best to leave
it alone.

Instead, support stack layout changes by "flattening" all possible stack
states (CFI) from parallel alternative code streams into a single set of
linear states.  The only necessary limitation is that CFI conflicts are
disallowed at all possible instruction boundaries.

For example, this scenario is allowed:

          Alt1                    Alt2                    Alt3

   0x00   CALL *pv_ops.save_fl    CALL xen_save_fl        PUSHF
   0x01                                                   POP %RAX
   0x02                                                   NOP
   ...
   0x05                           NOP
   ...
   0x07   <insn>

The unwind information for offset-0x00 is identical for all 3
alternatives.  Similarly offset-0x05 and higher also are identical (and
the same as 0x00).  However offset-0x01 has deviating CFI, but that is
only relevant for Alt3, neither of the other alternative instruction
streams will ever hit that offset.

This scenario is NOT allowed:

          Alt1                    Alt2

   0x00   CALL *pv_ops.save_fl    PUSHF
   0x01                           NOP6
   ...
   0x07   NOP                     POP %RAX

The problem here is that offset-0x7, which is an instruction boundary in
both possible instruction patch streams, has two conflicting stack
layouts.

[ The above examples were stolen from Peter Zijlstra. ]

The new flattened CFI array is used both for the detection of conflicts
(like the second example above) and the generation of linear ORC
entries.

BTW, another benefit of these changes is that, thanks to some related
cleanups (new fake nops and alt_group struct) objtool can finally be rid
of fake jumps, which were a constant source of headaches.

[1] https://lkml.kernel.org/r/20201111170536.arx2zbn4ngvjoov7@treble

Cc: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
2021-01-14 09:53:54 -06:00

262 lines
5.7 KiB
C

// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Copyright (C) 2017 Josh Poimboeuf <jpoimboe@redhat.com>
*/
#include <stdlib.h>
#include <string.h>
#include <linux/objtool.h>
#include <asm/orc_types.h>
#include <objtool/check.h>
#include <objtool/warn.h>
#include <objtool/endianness.h>
static int init_orc_entry(struct orc_entry *orc, struct cfi_state *cfi)
{
struct instruction *insn = container_of(cfi, struct instruction, cfi);
struct cfi_reg *bp = &cfi->regs[CFI_BP];
memset(orc, 0, sizeof(*orc));
orc->end = cfi->end;
if (cfi->cfa.base == CFI_UNDEFINED) {
orc->sp_reg = ORC_REG_UNDEFINED;
return 0;
}
switch (cfi->cfa.base) {
case CFI_SP:
orc->sp_reg = ORC_REG_SP;
break;
case CFI_SP_INDIRECT:
orc->sp_reg = ORC_REG_SP_INDIRECT;
break;
case CFI_BP:
orc->sp_reg = ORC_REG_BP;
break;
case CFI_BP_INDIRECT:
orc->sp_reg = ORC_REG_BP_INDIRECT;
break;
case CFI_R10:
orc->sp_reg = ORC_REG_R10;
break;
case CFI_R13:
orc->sp_reg = ORC_REG_R13;
break;
case CFI_DI:
orc->sp_reg = ORC_REG_DI;
break;
case CFI_DX:
orc->sp_reg = ORC_REG_DX;
break;
default:
WARN_FUNC("unknown CFA base reg %d",
insn->sec, insn->offset, cfi->cfa.base);
return -1;
}
switch (bp->base) {
case CFI_UNDEFINED:
orc->bp_reg = ORC_REG_UNDEFINED;
break;
case CFI_CFA:
orc->bp_reg = ORC_REG_PREV_SP;
break;
case CFI_BP:
orc->bp_reg = ORC_REG_BP;
break;
default:
WARN_FUNC("unknown BP base reg %d",
insn->sec, insn->offset, bp->base);
return -1;
}
orc->sp_offset = cfi->cfa.offset;
orc->bp_offset = bp->offset;
orc->type = cfi->type;
return 0;
}
static int write_orc_entry(struct elf *elf, struct section *orc_sec,
struct section *ip_rsec, unsigned int idx,
struct section *insn_sec, unsigned long insn_off,
struct orc_entry *o)
{
struct orc_entry *orc;
struct reloc *reloc;
/* populate ORC data */
orc = (struct orc_entry *)orc_sec->data->d_buf + idx;
memcpy(orc, o, sizeof(*orc));
orc->sp_offset = bswap_if_needed(orc->sp_offset);
orc->bp_offset = bswap_if_needed(orc->bp_offset);
/* populate reloc for ip */
reloc = malloc(sizeof(*reloc));
if (!reloc) {
perror("malloc");
return -1;
}
memset(reloc, 0, sizeof(*reloc));
insn_to_reloc_sym_addend(insn_sec, insn_off, reloc);
if (!reloc->sym) {
WARN("missing symbol for insn at offset 0x%lx",
insn_off);
return -1;
}
reloc->type = R_X86_64_PC32;
reloc->offset = idx * sizeof(int);
reloc->sec = ip_rsec;
elf_add_reloc(elf, reloc);
return 0;
}
struct orc_list_entry {
struct list_head list;
struct orc_entry orc;
struct section *insn_sec;
unsigned long insn_off;
};
static int orc_list_add(struct list_head *orc_list, struct orc_entry *orc,
struct section *sec, unsigned long offset)
{
struct orc_list_entry *entry = malloc(sizeof(*entry));
if (!entry) {
WARN("malloc failed");
return -1;
}
entry->orc = *orc;
entry->insn_sec = sec;
entry->insn_off = offset;
list_add_tail(&entry->list, orc_list);
return 0;
}
static unsigned long alt_group_len(struct alt_group *alt_group)
{
return alt_group->last_insn->offset +
alt_group->last_insn->len -
alt_group->first_insn->offset;
}
int orc_create(struct objtool_file *file)
{
struct section *sec, *ip_rsec, *orc_sec;
unsigned int nr = 0, idx = 0;
struct orc_list_entry *entry;
struct list_head orc_list;
struct orc_entry null = {
.sp_reg = ORC_REG_UNDEFINED,
.bp_reg = ORC_REG_UNDEFINED,
.type = UNWIND_HINT_TYPE_CALL,
};
/* Build a deduplicated list of ORC entries: */
INIT_LIST_HEAD(&orc_list);
for_each_sec(file, sec) {
struct orc_entry orc, prev_orc = {0};
struct instruction *insn;
bool empty = true;
if (!sec->text)
continue;
sec_for_each_insn(file, sec, insn) {
struct alt_group *alt_group = insn->alt_group;
int i;
if (!alt_group) {
if (init_orc_entry(&orc, &insn->cfi))
return -1;
if (!memcmp(&prev_orc, &orc, sizeof(orc)))
continue;
if (orc_list_add(&orc_list, &orc, sec,
insn->offset))
return -1;
nr++;
prev_orc = orc;
empty = false;
continue;
}
/*
* Alternatives can have different stack layout
* possibilities (but they shouldn't conflict).
* Instead of traversing the instructions, use the
* alt_group's flattened byte-offset-addressed CFI
* array.
*/
for (i = 0; i < alt_group_len(alt_group); i++) {
struct cfi_state *cfi = alt_group->cfi[i];
if (!cfi)
continue;
if (init_orc_entry(&orc, cfi))
return -1;
if (!memcmp(&prev_orc, &orc, sizeof(orc)))
continue;
if (orc_list_add(&orc_list, &orc, insn->sec,
insn->offset + i))
return -1;
nr++;
prev_orc = orc;
empty = false;
}
/* Skip to the end of the alt_group */
insn = alt_group->last_insn;
}
/* Add a section terminator */
if (!empty) {
orc_list_add(&orc_list, &null, sec, sec->len);
nr++;
}
}
if (!nr)
return 0;
/* Create .orc_unwind, .orc_unwind_ip and .rela.orc_unwind_ip sections: */
sec = find_section_by_name(file->elf, ".orc_unwind");
if (sec) {
WARN("file already has .orc_unwind section, skipping");
return -1;
}
orc_sec = elf_create_section(file->elf, ".orc_unwind", 0,
sizeof(struct orc_entry), nr);
if (!orc_sec)
return -1;
sec = elf_create_section(file->elf, ".orc_unwind_ip", 0, sizeof(int), nr);
if (!sec)
return -1;
ip_rsec = elf_create_reloc_section(file->elf, sec, SHT_RELA);
if (!ip_rsec)
return -1;
/* Write ORC entries to sections: */
list_for_each_entry(entry, &orc_list, list) {
if (write_orc_entry(file->elf, orc_sec, ip_rsec, idx++,
entry->insn_sec, entry->insn_off,
&entry->orc))
return -1;
}
if (elf_rebuild_reloc_section(file->elf, ip_rsec))
return -1;
return 0;
}