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Subject[PATCH RFC 00/19] perf tools and x86_64 KPTI entry trampolines
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Hi

Perf tools do not know about x86_64 KPTI entry trampolines - see example
below. These patches add a workaround, namely "perf tools: Workaround
missing maps for x86_64 KPTI 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 (17):
kallsyms: Simplify update_iter_mod()
x86: kcore: Give entry trampolines all the same offset in kcore
perf tools: Use the _stest symbol to identify the kernel map when loading kcore
perf tools: Fix kernel_start for KPTI on x86_64
perf tools: Workaround missing maps for x86_64 KPTI entry trampolines
perf tools: Fix map_groups__split_kallsyms() for entry trampoline symbols
perf tools: Allow for special kernel maps
perf tools: Create maps for x86_64 KPTI entry trampolines
perf tools: Synthesize and process mmap events for x86_64 KPTI 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_64 entry trampoline sections
perf buildid-cache: kcore_copy: Amend the offset of sections that remap kernel text

Alexander Shishkin (2):
kallsyms, x86: Export addresses of syscall trampolines
x86: Add entry trampolines to kcore

arch/x86/mm/cpu_entry_area.c | 28 +++++
fs/proc/kcore.c | 7 +-
include/linux/kcore.h | 13 ++
kernel/kallsyms.c | 46 ++++---
tools/perf/util/event.c | 92 +++++++++++++-
tools/perf/util/machine.c | 288 ++++++++++++++++++++++++++++++++++++++++++-
tools/perf/util/machine.h | 6 +
tools/perf/util/map.c | 22 +++-
tools/perf/util/map.h | 15 ++-
tools/perf/util/symbol-elf.c | 209 ++++++++++++++++++++++++++-----
tools/perf/util/symbol.c | 65 +++++++---
11 files changed, 709 insertions(+), 82 deletions(-)


Regards
Adrian

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