[PATCH V4 0/3] perf tools and x86 PTI entry trampolines

From: Adrian Hunter
Date: Wed Jun 06 2018 - 08:55:38 EST

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Hi

Here is V4 of patches to support x86 PTI entry trampolines in perf tools.

Patches also here:
http://git.infradead.org/users/ahunter/linux-perf.git/shortlog/refs/heads/perf-tools-kpti-v4
git://git.infradead.org/users/ahunter/linux-perf.git perf-tools-kpti-v4

V3 patches also here:
http://git.infradead.org/users/ahunter/linux-perf.git/shortlog/refs/heads/perf-tools-kpti-v3
git://git.infradead.org/users/ahunter/linux-perf.git perf-tools-kpti-v3

V2 patches also here:
http://git.infradead.org/users/ahunter/linux-perf.git/shortlog/refs/heads/perf-tools-kpti-v2
git://git.infradead.org/users/ahunter/linux-perf.git perf-tools-kpti-v2

V1 patches also here:
http://git.infradead.org/users/ahunter/linux-perf.git/shortlog/refs/heads/perf-tools-kpti-v1
git://git.infradead.org/users/ahunter/linux-perf.git perf-tools-kpti-v1

Changes Since V3:
kallsyms: Simplify update_iter_mod()
Added comment
Added Andi's Ack

kallsyms, x86: Export addresses of PTI entry trampolines
Expanded commit message
Used for_each_possible_cpu()
Added Andi's Ack even though logic changed slightly

x86: Add entry trampolines to kcore
Re-based
Added Andi's Ack

perf tools: Add machine__nr_cpus_avail()
perf tools: Workaround missing maps for x86 PTI entry trampolines
perf tools: Fix map_groups__split_kallsyms() for entry trampoline symbols
perf tools: Allow for extra kernel maps
perf tools: Create maps for x86 PTI entry trampolines
perf tools: Synthesize and process mmap events for x86 PTI entry trampolines
perf buildid-cache: kcore_copy: Keep phdr data in a list
perf buildid-cache: kcore_copy: Keep a count of phdrs
perf buildid-cache: kcore_copy: Calculate offset from phnum
perf buildid-cache: kcore_copy: Layout sections
perf buildid-cache: kcore_copy: Iterate phdrs
perf buildid-cache: kcore_copy: Get rid of kernel_map
perf buildid-cache: kcore_copy: Copy x86 PTI entry trampoline sections
perf buildid-cache: kcore_copy: Amend the offset of sections that remap kernel text
Dropped because they have been applied

Changes Since V2:

x86: Add entry trampolines to kcore
x86: kcore: Give entry trampolines all the same offset in kcore
Combined into a single patch
Added comment
Expand commit message

perf tools: Add machine__is() to identify machine arch
Dropped because it has been applied

perf tools: Fix kernel_start for PTI on x86
Dropped because it has been applied

Changes Since V1:

perf tools: Use the _stest symbol to identify the kernel map when loading kcore
Dropped because it has been applied

perf tools: Add machine__is() to identify machine arch
New patch

perf tools: Fix kernel_start for PTI on x86
Moved definition of machine__is() to a separate patch

perf tools: Add machine__nr_cpus_avail()
New patch

perf tools: Workaround missing maps for x86 PTI entry trampolines
Use machine__nr_cpus_avail()

perf tools: Create maps for x86 PTI entry trampolines
Re-based

Changes Since RFC:

Change description 'x86_64 KPTI' to 'x86 PTI'

Rename 'special' kernel map to 'extra' kernel map etc

kallsyms: Simplify update_iter_mod()
Expand commit message

perf tools: Fix kernel_start for PTI on x86
Amend machine__is() to check if machine is NULL

perf tools: Workaround missing maps for x86 PTI entry trampolines
Simplify find_entry_trampoline()
Add comment before struct extra_kernel_map /* Kernel-space
maps for symbols that are outside the main kernel map and
module maps */

perf tools: Create maps for x86 PTI entry trampolines
Move code presently only used by x86_64 into arch

perf tools: Synthesize and process mmap events for x86 PTI entry
trampolines
Fix spelling 'kernal' -> 'kernel'
Rename 'special' kernel map to 'extra' kernel map etc
Move code presently only used by x86_64 into arch

perf buildid-cache: kcore_copy: Keep phdr data in a list
Expand commit message
Rename 'list' -> 'node'

perf buildid-cache: kcore_copy: Get rid of kernel_map
Expand commit message
Add phdr_data__new()
Rename 'kcore_copy__new_phdr' -> 'kcore_copy_info__addnew'

Original Cover email:

Perf tools do not know about x86 PTI entry trampolines - see example
below. These patches add a workaround, namely "perf tools: Workaround
missing maps for x86 PTI entry trampolines", which has the limitation
that it hard codes the addresses. Note that the workaround will work for
old kernels and old perf.data files, but not for future kernels if the
trampoline addresses are ever changed.

At present, perf tools uses /proc/kallsyms to construct a memory map for
the kernel. Recording such a map in the perf.data file is necessary to
deal with kernel relocation and KASLR.

While it is reasonable on its own terms, to add symbols for the trampolines
to /proc/kallsyms, the motivation here is to have perf tools use them to
create memory maps in the same fashion as is done for the kernel text.

So the first 2 patches add symbols to /proc/kallsyms for the trampolines:

kallsyms: Simplify update_iter_mod()
kallsyms, x86: Export addresses of syscall trampolines

perf tools have the ability to use /proc/kcore (in conjunction with
/proc/kallsyms) as the kernel image. So the next 2 patches add program
headers for the trampolines to the kcore ELF:

x86: Add entry trampolines to kcore
x86: kcore: Give entry trampolines all the same offset in kcore

It is worth noting that, with the kcore changes alone, perf tools require
no changes to recognise the trampolines when using /proc/kcore.

Similarly, if perf tools are used with a matching kallsyms only (by denying
access to /proc/kcore or a vmlinux image), then the kallsyms patches are
sufficient to recognise the trampolines with no changes needed to the
tools.

However, in the general case, when using vmlinux or dealing with
relocations, perf tools needs memory maps for the trampolines. Because the
kernel text map is constructed as a special case, using the same approach
for the trampolines means treating them as a special case also, which
requires a number of changes to perf tools, and the remaining patches deal
with that.

Example: make a program that does lots of small syscalls e.g.

$ cat uname_x_n.c

#include <sys/utsname.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
long n = argc > 1 ? strtol(argv[1], NULL, 0) : 0;
struct utsname u;

while (n--)
uname(&u);

return 0;
}

and then:

sudo perf record uname_x_n 100000
sudo perf report --stdio

Before the changes, there are unknown symbols:

# Overhead Command Shared Object Symbol
# ........ ......... ................ ..................................
#
41.91% uname_x_n [kernel.vmlinux] [k] syscall_return_via_sysret
19.22% uname_x_n [kernel.vmlinux] [k] copy_user_enhanced_fast_string
18.70% uname_x_n [unknown] [k] 0xfffffe00000e201b
4.09% uname_x_n libc-2.19.so [.] __GI___uname
3.08% uname_x_n [kernel.vmlinux] [k] do_syscall_64
3.02% uname_x_n [unknown] [k] 0xfffffe00000e2025
2.32% uname_x_n [kernel.vmlinux] [k] down_read
2.27% uname_x_n ld-2.19.so [.] _dl_start
1.97% uname_x_n [unknown] [k] 0xfffffe00000e201e
1.25% uname_x_n [kernel.vmlinux] [k] up_read
1.02% uname_x_n [unknown] [k] 0xfffffe00000e200c
0.99% uname_x_n [kernel.vmlinux] [k] entry_SYSCALL_64
0.16% uname_x_n [kernel.vmlinux] [k] flush_signal_handlers
0.01% perf [kernel.vmlinux] [k] native_sched_clock
0.00% perf [kernel.vmlinux] [k] native_write_msr

After the changes there are not:

# Overhead Command Shared Object Symbol
# ........ ......... ................ ..................................
#
41.91% uname_x_n [kernel.vmlinux] [k] syscall_return_via_sysret
24.70% uname_x_n [kernel.vmlinux] [k] entry_SYSCALL_64_trampoline
19.22% uname_x_n [kernel.vmlinux] [k] copy_user_enhanced_fast_string
4.09% uname_x_n libc-2.19.so [.] __GI___uname
3.08% uname_x_n [kernel.vmlinux] [k] do_syscall_64
2.32% uname_x_n [kernel.vmlinux] [k] down_read
2.27% uname_x_n ld-2.19.so [.] _dl_start
1.25% uname_x_n [kernel.vmlinux] [k] up_read
0.99% uname_x_n [kernel.vmlinux] [k] entry_SYSCALL_64
0.16% uname_x_n [kernel.vmlinux] [k] flush_signal_handlers
0.01% perf [kernel.vmlinux] [k] native_sched_clock
0.00% perf [kernel.vmlinux] [k] native_write_msr

Adrian Hunter (2):
kallsyms: Simplify update_iter_mod()
x86: Add entry trampolines to kcore

Alexander Shishkin (1):
kallsyms, x86: Export addresses of PTI entry trampolines

arch/x86/mm/cpu_entry_area.c | 33 ++++++++++++++++++++++++++++
fs/proc/kcore.c | 7 ++++--
include/linux/kcore.h | 13 +++++++++++
kernel/kallsyms.c | 51 ++++++++++++++++++++++++++++++++------------
4 files changed, 88 insertions(+), 16 deletions(-)

Regards
Adrian

Next message: Adrian Hunter: "[PATCH V4 1/3] kallsyms: Simplify update_iter_mod()"
Previous message: Rob Herring: "Re: [PATCH v2 5/5] venus: register separate driver for firmware device"
Next in thread: Adrian Hunter: "[PATCH V4 1/3] kallsyms: Simplify update_iter_mod()"
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