Implement function cgroupid(path) -> cgroup id #170

alban · 2018-10-11T14:31:16Z

This PR is the second part to implement #150.

Note: it works fine on my Fedora laptop when compiled with:

LLVM_DIR=/usr/lib64/llvm cmake . && make all

But the Docker build stopped building. The alpine builder image is based on musl libc instead of glibc and it does not support name_to_handle_at. I tried to switch back to the ubuntu builder image but I got the same error as in this email from linux.debian.bugs.dist and I couldn't fix it.

I added a test in tests/codegen.cpp for the builtin cgroup but I am not sure how to check the generated instructions. I assume it is fine because it works when I test manually.

I didn't add a test for the function cgroupid() because it does not generate any interesting BPF instructions but just calls name_to_handle_at() in userspace. The result of that is depending on the local system. With systemd configured in cgroup hybrid mode, cgroup-v2 would be mounted in /sys/fs/cgroup/unified but it could be mounted differently in other systemd configuration.

alban · 2018-10-11T15:42:51Z

I also tried to build from a Fedora Docker image, without success (build log).

brendangregg · 2018-10-11T16:45:50Z

Yes, we think there's a different issue with Docker builds at the moment. This patch looks good.

brendangregg · 2018-10-11T16:58:44Z

Just noticed: this needs an addition to the reference guide as well. Thanks!

ajor · 2018-10-11T18:10:09Z

The Fedora Docker build errors are because it’s using LLVM 7 - see #149

The Docker errors we’ve got at the moment are limited to string comparisons so I’d rather not completely break the Alpine build

ajor · 2018-10-11T18:35:43Z

src/bpftrace.cpp

+  uint64_t cgid;
+  int err, mount_id;
+  file_handle *handle = reinterpret_cast<struct file_handle *>(
+    malloc(sizeof(file_handle) + sizeof(uint64_t)));


Can handle just be allocated on the stack instead here?

Oh, I see it needs more space than the struct now...
Could we do something like create our own wrapper struct with the regular file_handle and a uint64_t inside to avoid the dynamic allocation then? Or use smart pointers - I just don't like having malloc and free in here

ok, I'll rewrite it to put it on the stack.

ajor · 2018-10-11T19:12:58Z

name_to_handle_at was added to musl a month ago but there was a release a week before that so I don't think we can just wait for them to put out another release. I think Alpine is necessary as the base Docker image because it was the only distro I found which packaged libclang as a static library.

Maybe we could just #ifdef this code out so it's not available from Alpine/Docker builds until the next release of musl

krnowak · 2018-10-12T11:08:51Z

src/ast/semantic_analyser.cpp

@@ -254,6 +254,14 @@ void SemanticAnalyser::visit(Call &call)
    }
    call.type = SizedType(Type::integer, 8);
  }
+  else if (call.func == "cgroupid") {
+    if (check_nargs(call, 1)) {
+      if (check_arg(call, Type::string, 0, true)) {


This if is pointless, no? So just having check_arg(call, …); should be enough.

I was reproducing the coding style from above

Appreciate you sticking to the file's coding style, but I think in this case that code is just wrong

krnowak · 2018-10-12T11:10:37Z

src/bpftrace.cpp

+{
+  uint64_t cgid;
+  int err, mount_id;
+  file_handle *handle = reinterpret_cast<struct file_handle *>(


The struct keyword inside reinterpret cast is unnecessary.

ok, I'll remove it.

alban

I had a look at the alpine:edge and alpine:latest docker images but they still don't provide name_to_handle_at. I'll add #ifdef for now, as suggested.

alban · 2018-10-12T12:55:44Z

src/ast/semantic_analyser.cpp

@@ -254,6 +254,14 @@ void SemanticAnalyser::visit(Call &call)
    }
    call.type = SizedType(Type::integer, 8);
  }
+  else if (call.func == "cgroupid") {
+    if (check_nargs(call, 1)) {
+      if (check_arg(call, Type::string, 0, true)) {


I was reproducing the coding style from above

alban · 2018-10-12T12:56:08Z

src/bpftrace.cpp

+{
+  uint64_t cgid;
+  int err, mount_id;
+  file_handle *handle = reinterpret_cast<struct file_handle *>(


ok, I'll remove it.

alban · 2018-10-12T12:56:54Z

src/bpftrace.cpp

+  uint64_t cgid;
+  int err, mount_id;
+  file_handle *handle = reinterpret_cast<struct file_handle *>(
+    malloc(sizeof(file_handle) + sizeof(uint64_t)));


ok, I'll rewrite it to put it on the stack.

krnowak · 2018-10-15T18:52:08Z

Updated the PR, I think I addresses all the review issues. But running build.sh failed here, though:

~/projects/kinvolk/bpftrace
-- The C compiler identification is GNU 6.4.0
-- The CXX compiler identification is GNU 6.4.0
-- Check for working C compiler: /usr/bin/cc
CMake Error: Generator: execution of make failed. Make command was: "/usr/bin/gmake" "cmTC_0204d/fast"
-- Check for working C compiler: /usr/bin/cc -- broken
CMake Error at /usr/share/cmake/Modules/CMakeTestCCompiler.cmake:52 (message):
  The C compiler

    "/usr/bin/cc"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /home/kv/projects/kinvolk/bpftrace/CMakeFiles/CMakeTmp
    
    Run Build Command:"/usr/bin/gmake" "cmTC_0204d/fast"
    No such file or directory
    Generator: execution of make failed. Make command was: "/usr/bin/gmake" "cmTC_0204d/fast"
    

  

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- Configuring incomplete, errors occurred!
See also "/home/kv/projects/kinvolk/bpftrace/CMakeFiles/CMakeOutput.log".
See also "/home/kv/projects/kinvolk/bpftrace/CMakeFiles/CMakeError.log".

krnowak · 2018-10-15T19:01:13Z

Uh, I still need to add it to the reference guide, I haven't noticed it before.

brendangregg · 2018-10-15T20:19:17Z

I doubt your changes broke build.sh, that's likely a separate issue we need to fix. I've been doing manual builds on ubuntu as shown in install.md.

I'd suggest improving this error message:

# bpftrace -e 'tracepoint:syscalls:sys_enter_openat /cgroup == cgroupid("/sys/fs/cgroup/unified/mycg")/ { printf("%s\n", str(args->filename)); }'
terminate called after throwing an instance of 'std::runtime_error'
  what():  name_to_handle_at() failed!
Aborted (core dumped)

Instead of what(): name_to_handle_at() failed!, I think it'd be better to say: cgroup /sys/fs/cgroup/unified/mycg not found.

This also core dumps (it doesn't need to), but that's something we can clean up separately along with the other normal error paths that use throw when they probably should use something else. So right now I'd just change the text.

ajor · 2018-10-15T20:43:43Z

build.sh still works for me when I check out your branch. Not sure what's causing the problem you're seeing, but I don't think these changes are responsible

We will wrap the file_handle struct in the next commit. The struct has a flexible array member, which is not supported by C++. Compiler may complain about using it when allocated on stack, even indirectly as a member of a struct. I'm not sure if using this kind of types is even a defined behavior…

Currently used image of alpine has a version of musl libc that does not have name_to_handle_at function that is required to resolve the cgroup id. In such case we just use an implementation that says that resolving cgroupid is not supported.

Mention the `cgroup` variable and the `cgroupid` function alongside an example that uses them both.

krnowak · 2018-10-16T12:20:57Z

Updated the PR - changed the error message (I think it would be nice to consume errno there too), updated the reference to mention both cgroup variable and cgroupid function. Apart from that, build.sh started working again, after I dropped all the images and refetched everything again. I guess something got updated in the meantime or I was using some old broken image of something (alpine?).

brendangregg · 2018-10-16T15:02:47Z

thanks!

ajor reviewed Oct 11, 2018

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krnowak reviewed Oct 12, 2018

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alban commented Oct 12, 2018

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krnowak force-pushed the alban/func_cgroupid branch from 4ef61ef to 86f6ca4 Compare October 15, 2018 18:47

krnowak and others added 4 commits October 16, 2018 14:09

Implement function cgroupid()

64abee2

Update reference documentation

5db00d7

Mention the `cgroup` variable and the `cgroupid` function alongside an example that uses them both.

krnowak force-pushed the alban/func_cgroupid branch from 86f6ca4 to 5db00d7 Compare October 16, 2018 12:18

brendangregg merged commit 2a906eb into bpftrace:master Oct 16, 2018

krnowak mentioned this pull request Oct 16, 2018

Use syscall when name_to_handle_at is not available #185

Merged

chrahunt mentioned this pull request Mar 15, 2020

Implement filter for cgroups #150

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Implement function cgroupid(path) -> cgroup id #170

Implement function cgroupid(path) -> cgroup id #170

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Implement function cgroupid(path) -> cgroup id #170

Implement function cgroupid(path) -> cgroup id #170

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