linux/tools
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
..
accounting
arch x86/insn: Fix vector instruction decoding on big endian cross-compiles 2021-01-13 18:13:17 -06:00
bootconfig tools/bootconfig: Store size and checksum in footer as le32 2020-11-30 23:22:11 -05:00
bpf bpftool: Fix compilation failure for net.o with older glibc 2021-01-06 15:27:38 -08:00
build Merge remote-tracking branch 'torvalds/master' into perf/core 2020-12-17 14:37:24 -03:00
cgroup blk-iocost: update iocost_monitor.py 2020-09-01 19:38:33 -06:00
debugging docs: Update documentation to reflect what TAINT_CPU_OUT_OF_SPEC means 2020-12-08 10:53:58 -07:00
edid
firewire
firmware
gpio tools: gpio: add option to report wall-clock time to gpio-event-mon 2020-12-05 23:22:48 +01:00
hv tools: hv: change http to https in hv_kvp_daemon.c 2020-07-06 10:46:23 +00:00
iio iio: add IIO_MOD_O2 modifier 2020-08-22 10:53:12 +01:00
include tools headers UAPI: Sync kvm.h headers with the kernel sources 2020-12-24 09:24:19 -03:00
io_uring tools/io_uring: fix compile breakage 2020-09-21 07:50:58 -06:00
kvm/kvm_stat tools/kvm_stat: Exempt time-based counters 2020-12-11 19:18:51 -05:00
laptop change email address for Pali Rohár 2020-04-10 15:36:22 -07:00
leds
lib Merge remote-tracking branch 'torvalds/master' into perf/core 2020-12-17 14:37:24 -03:00
memory-model tools/memory-model: Label MP tests' producers and consumers 2020-11-06 17:25:17 -08:00
objtool objtool: Support stack layout changes in alternatives 2021-01-14 09:53:54 -06:00
pci
pcmcia
perf perf probe: Fix memory leak when synthesizing SDT probes 2020-12-24 10:52:10 -03:00
power Kbuild updates for v5.11 2020-12-22 14:02:39 -08:00
scripts tools: Factor HOSTCC, HOSTLD, HOSTAR definitions 2020-11-11 12:18:22 -08:00
spi spi: tools: Make default_tx/rx and input_tx static 2020-06-11 16:27:24 +01:00
testing linux-kselftest-kunit-fixes-5.11-rc3 2021-01-08 17:18:50 -08:00
thermal/tmon - Convert tsens configuration DT binding to yaml (Rajeshwari) 2020-04-07 20:00:16 -07:00
time
usb tools: usb: move to tools buildsystem 2020-08-19 14:11:44 +02:00
virtio tools/virtio: add barrier for aarch64 2020-12-18 16:14:30 -05:00
vm mm: Add PG_arch_2 page flag 2020-09-04 12:46:06 +01:00
wmi
Makefile bpf: Compile resolve_btfids tool at kernel compilation start 2020-07-13 10:42:02 -07:00