There is no /proc/self/ns/mnt in my linux

[Hoegje]

Maybe I am running an older kernel, but I even on newer Centos versions it seems there is no namespace folder in the proc filesystem. An earlier comment on issue #49 (#49 (comment)) proposed the second pull request to be used in this case, in stead of using the /proc/self/ns folder to go through the /proc/PID/root folder. That one can be found on my CentOS boxes as well.

Original Pull request:
rbspy/rbspy#74 (comment)

Strace of my issue:
readlink("/proc/30203/exe", "/opt/python-3-dbg/bin/python3.6", 256) = 31
open("/proc/30203/maps", O_RDONLY|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
read(3, "00400000-00401000 r-xp 00000000 ", 32) = 32
read(3, "08:02 55117221 ", 32) = 32
read(3, " /opt/python-3-dbg/bin/p"..., 64) = 64
read(3, "00001000 08:02 55117221 "..., 128) = 128
read(3, " [heap"..., 256) = 256
read(3, "rw-p 00007000 08:02 28024844 "..., 512) = 512
read(3, " /lib64/libcrypt-2"..., 1024) = 1024
read(3, " /lib64/ld-2.12.so\n3f10a22000-3"..., 2048) = 2048
read(3, "f12000000-3f12015000 r-xp 000000"..., 4096) = 4096
read(3, "0.1\n3f1f00a000-3f1f209000 ---p 0"..., 8192) = 4091
read(3, "7fdf23600000-7fdf24000000 rw-p 0"..., 4101) = 3994
read(3, "7fdf3cd89000-7fdf3cd8c000 rw-p 0"..., 107) = 107
brk(0x1a8c000) = 0x1a8c000
read(3, "b-dynload/_sqlite3.cpython-36dm-"..., 16384) = 4137
read(3, "7fdf3e46d000-7fdf3e46f000 r-xp 0"..., 12247) = 3949
read(3, "7fdf3f657000-7fdf3f65d000 r-xp 0"..., 8298) = 4002
read(3, "7fdf46609000-7fdf4660a000 rw-s 0"..., 4296) = 2720
read(3, "", 1576) = 0
brk(0x1a90000) = 0x1a90000
brk(0x1a97000) = 0x1a97000
brk(0x1aa5000) = 0x1aa5000
close(3) = 0
open("/proc/30203/root/opt/python-3-dbg/bin/python3.6", O_RDONLY|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=12681, ...}) = 0
mmap(NULL, 12681, PROT_READ, MAP_SHARED, 3, 0) = 0x7fec28a52000
munmap(0x7fec28a52000, 12681) = 0
close(3) = 0
brk(0x1aa9000) = 0x1aa9000
brk(0x1ab0000) = 0x1ab0000
brk(0x1ab5000) = 0x1ab5000
brk(0x1aba000) = 0x1aba000
readlink("/proc/self/ns/mnt", 0x1ab9c60, 256) = -1 ENOENT (No such file or directory)
write(2, "Error: ", 7Error: ) = 7
write(2, "No such file or directory", 25No such file or directory) = 25
write(2, " (os error ", 11 (os error ) = 11
write(2, "2", 12) = 1
write(2, ")", 1)) = 1
write(2, "\n", 1
) = 1
write(2, "\n", 1
) = 1

It seems that it first goes for the /proc/PID/root folder, but then afterwards it still tries to set the namespace and it fails there.

[benfred]

I think the only place that tries to access /proc/*/ns/mnt is the check to see if we're running in a docker container:

py-spy/src/python_spy.rs

Lines 838 to 844 in 107d09a

	#[cfg(target_os="linux")]
	let dockerized = {
	let target_ns_filename = format!("/proc/{}/ns/mnt", process.pid);
	let self_mnt = std::fs::read_link("/proc/self/ns/mnt")?;
	let target_mnt = std::fs::read_link(&target_ns_filename)?;
	self_mnt != target_mnt
	};

The rest of file accesses are through /proc/PID/root on linux afaict

If the ns/mnt directories don't exist, this check will cause py-spy initialization to fail and the errors you saw =( I think we should change this check to ignore failures, and assume that we aren't running under docker if those folders don't exist

[benfred]

That last commit should fix - if you want to try it out yourself you can download the wheels from the build artifacts (https://github.com/benfred/py-spy/runs/1729266514) and install py-spy from there.

[Hoegje]

I think using the wheels.zip from the build you linked, I get a bit further now. I will have to look as to why it fails this time with:

~/.local/bin/py-spy top --pid 15330
Error: Failed to find a python interpreter in the .data section

Thanks for the help on this one already.