Fallback to /proc/PID/mem when process_vm_readv not in kernel

[cakemanny]

Resolves #238

Describe your changes
I've added an alternative implementation of ProcessMemoryManager::readChunk that uses the relevant /proc/PID/mem file and is fallen back to when process_vm_readv fails with ENOSYS.
Once the /proc/PID/mem file is open, subsequent reads are done only from there.

Testing performed

On my dev instance where my kernel does have CONFIG_CROSS_MEMORY_ATTACH set, I did various tests with small source changes. Such as forcing the fallback implementation to always be used.

Details

diff --git a/src/pystack/_pystack/mem.cpp b/src/pystack/_pystack/mem.cpp
index e25c69f..5deda3d 100644
--- a/src/pystack/_pystack/mem.cpp
+++ b/src/pystack/_pystack/mem.cpp
@@ -229,11 +229,15 @@ ProcessMemoryManager::ProcessMemoryManager(pid_t pid)
 ssize_t
 ProcessMemoryManager::readChunk(remote_addr_t addr, size_t len, char* dst) const
 {
+#if 0
     if (d_memfile.is_open()) {
         return readChunkThroughMemFile(addr, len, dst);
     } else {
         return readChunkDirect(addr, len, dst);
     }
+#else
+        return readChunkThroughMemFile(addr, len, dst);
+#endif
 }
 
 ssize_t

dan:~/src/pystack (support-missing-process_vm_readv) % python3 -c $'import time\nwhile True: time.sleep(1)' &
[1] 27161
dan:~/src/pystack (support-missing-process_vm_readv) % pystack remote $(pgrep python3)                       
Traceback for thread 27161 (python3) [] (most recent call last):
    (Python) File "<string>", line 2, in <module>

dan:~/src/pystack (support-missing-process_vm_readv) % pytest
===================================== test session starts =====================================
platform linux -- Python 3.11.2, pytest-8.3.5, pluggy-1.6.0
rootdir: /home/dan/src/pystack
...
=============================== 459 passed, 3 skipped in 15.75s ===============================

I also checked what the various errors might look like, say if the file didn't exist

@@ -273,7 +277,7 @@ ssize_t
 ProcessMemoryManager::readChunkThroughMemFile(remote_addr_t addr, size_t len, char* dst) const
 {
     if (!d_memfile.is_open()) {
-        std::string filepath = "/proc/" + std::to_string(d_pid) + "/mem";
+        std::string filepath = "/proc/" + std::to_string(d_pid) + "/memXXX";
         d_memfile.open(filepath, std::ifstream::binary);
         if (!d_memfile) {
             LOG(ERROR) << "Failed to open file " << filepath;

dan:~/src/pystack (support-missing-process_vm_readv) % pystack remote $(pgrep python3)
ERROR(process_remote): Failed to open file /proc/27161/memXXX
💀 Engine error: No such file or directory 💀

dan:~/src/pystack (support-missing-process_vm_readv) %

Once I was pretty happy that it wasn't too awful, I spun up a flatcar VM and tested the happy path there.

Details (Probably too many)

Per https://www.flatcar.org/docs/latest/setup/releases/verify-images/

curl -L -O https://www.flatcar.org/security/image-signing-key/Flatcar_Image_Signing_Key.asc
gpg --import --keyid-format LONG Flatcar_Image_Signing_Key.asc

cat  << 'EOF' > ssh_config
Host flatcar
HostName localhost
Port 2222
User core
StrictHostKeyChecking no
UserKnownHostsFile /dev/null
EOF
cat << 'EOF' > acquire.sh 
#!/bin/sh

set -eu

#current=4152.2.3
current=current

base_name=flatcar_production_qemu_uefi

for suff in .sh _image.img _efi_code.fd _efi_vars.fd; do
  wget https://lts.release.flatcar-linux.net/arm64-usr/$current/${base_name}${suff}
  wget https://lts.release.flatcar-linux.net/arm64-usr/$current/${base_name}${suff}.sig
  gpg --verify ${base_name}${suff}.sig
done

chmod +x ${base_name}.sh
EOF

sh ./acquire.sh

patch <<'EOF'
diff --git a/flatcar_production_qemu_uefi.sh b/flatcar_production_qemu_uefi.sh
index e97ba31..0e31ba9 100755
--- a/flatcar_production_qemu_uefi.sh
+++ b/flatcar_production_qemu_uefi.sh
@@ -220,6 +220,8 @@ else
             set -- -machine q35 -cpu kvm64 -smp 1 -nographic "$@" ;;
         arm64-usr+aarch64)
             set -- -machine virt,accel=kvm,gic-version=3 -cpu host -smp "${VM_NCPUS}" -nographic "$@" ;;
+        arm64-usr+arm64)
+            set -- -machine virt,accel=hvf -cpu host -smp "${VM_NCPUS}" -nographic "$@" ;;
         arm64-usr+*)
             if test "${VM_NCPUS}" -gt 4 ; then
                 VM_NCPUS=4
EOF

./flatcar_production_qemu_uefi.sh -nographic

ssh -F ssh_config flatcar

core@localhost ~ $ docker run -it --rm python:3.12 bash
Unable to find image 'python:3.12' locally
...
Status: Downloaded newer image for python:3.12
root@946ac0b130f3:/# # fix broken dns
root@946ac0b130f3:/# newresolve=$(sed $'/nameserver/i\\\nnameserver 1.1.1.1' /etc/resolv.conf); echo "$newresolve" > /etc/resolv.conf
root@946ac0b130f3:/# apt-get update && apt-get install libdw-dev libelf-dev pkg-config
...
root@946ac0b130f3:/# python -m pip install "pystack @ git+https://github.com/cakemanny/pystack.git@support-missing-process_vm_readv"
...
root@dce305e0c604:/# pystack
usage: pystack [-h] [-v] [--version] [--no-color] {remote,core} ...
pystack: error: the following arguments are required: command
root@dce305e0c604:/# python3 -c $'import time\nwhile True: time.sleep(1)' &
[1] 260
root@dce305e0c604:/# pystack remote 260
Traceback for thread 260 (python3) [] (most recent call last):
    (Python) File "<string>", line 2, in <module>

root@dce305e0c604:/# pystack remote 260 -vv 2>&1| grep falling
DEBUG(process_remote): process_vm_readv not compiled in kernel, falling back to /proc/PID/mem
root@dce305e0c604:/#

Additional context
I've not written any C++ for about 14 years, so I've probably missed out on some cleverer or prettier ways of doing this.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 13.04348% with 20 lines in your changes missing coverage. Please review.

Project coverage is 82.41%. Comparing base (ca83721) to head (3ef98d2).

Files with missing lines	Patch %	Lines
src/pystack/_pystack/mem.cpp	13.04%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
- Coverage   82.66%   82.41%   -0.26%     
==========================================
  Files          46       46              
  Lines        6249     6272      +23     
  Branches      458      465       +7     
==========================================
+ Hits         5166     5169       +3     
- Misses       1083     1103      +20     

Flag	Coverage Δ
cpp	82.41% <13.04%> (-0.26%)	⬇️
python_and_cython	82.41% <13.04%> (-0.26%)	⬇️

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