Messages in this thread | | | From | Adrian Hunter <> | Subject | [PATCH RFC 00/19] perf tools and x86_64 KPTI entry trampolines | Date | Wed, 9 May 2018 14:43:29 +0300 |
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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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