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https://github.com/edolstra/dwarffs
A FUSE filesystem that allows tools like gdb to look up debug info files via HTTP
https://github.com/edolstra/dwarffs
Last synced: 3 days ago
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A FUSE filesystem that allows tools like gdb to look up debug info files via HTTP
- Host: GitHub
- URL: https://github.com/edolstra/dwarffs
- Owner: edolstra
- License: gpl-3.0
- Created: 2017-07-06T16:43:44.000Z (over 7 years ago)
- Default Branch: master
- Last Pushed: 2024-12-02T13:32:34.000Z (about 2 months ago)
- Last Synced: 2025-01-11T05:12:20.274Z (10 days ago)
- Language: C++
- Homepage:
- Size: 87.9 KB
- Stars: 201
- Watchers: 12
- Forks: 22
- Open Issues: 7
-
Metadata Files:
- Readme: README.md
- License: COPYING
Awesome Lists containing this project
README
# dwarffs
`dwarffs` is a FUSE-based filesystem that fetches DWARF debug info
files automatically from `cache.nixos.org`, based on the build ID
embedded in ELF executables and libraries.
## Usage
### NixOS
To enable `dwarffs`, add it to your NixOS configuration flake like
this:
```nix
{
inputs.dwarffs.url = "github:edolstra/dwarffs";
outputs = { self, nixpkgs, dwarffs }: {
nixosConfigurations.my-machine = nixpkgs.lib.nixosSystem {
modules =
[ dwarffs.nixosModules.dwarffs
# ... other configuration ...
];
};
};
}
```
You may also need an unstable version of Nix, which can be obtained
from the `nix` flake:
```nix
{
inputs.dwarffs.url = "github:edolstra/dwarffs";
inputs.nix.url = "github:NixOS/nix";
outputs = { self, nixpkgs, dwarffs, nix }: {
nixosConfigurations.my-machine = nixpkgs.lib.nixosSystem {
modules =
[ dwarffs.nixosModules.dwarffs
{ nixpkgs.overlays = [ nix.overlays.default ]; }
# ... other configuration ...
];
};
};
}
```
This creates an automount unit on `/run/dwarffs`. It also sets the
environment variable `NIX_DEBUG_INFO_DIRS` to `/run/dwarffs`. The
`gdb` and `elfutils` packages in Nixpkgs have been patched to use that
environment variable, allowing them to use `dwarffs` automatically.
For example, to get line number information from `eu-stack` for a
running process:
```
$ eu-stack -p 28080 -s
PID 28080 - process
TID 28080:
#0 0x00007f1ee343f99d read
../sysdeps/unix/syscall-template.S:84
#1 0x00007f1ee4147864 nix::FdSource::readUnbuffered(unsigned char*, unsigned long)
/nix/store/50jw5m7lda3rylirxyly9diy55lh149z-glibc-2.25-49-dev/include/bits/unistd.h:44
#2 0x00007f1ee4146d66 nix::BufferedSource::read(unsigned char*, unsigned long)
src/libutil/serialise.cc:94
...
```
Similarly, to get line number information from a core dump:
```
$ eu-stack --core ./core -s
PID 22154 - core
TID 22154:
#0 0x00007fd98db9bfcd __read
../sysdeps/unix/syscall-template.S:84
#1 0x000000000041cf13 linenoise
/nix/store/50jw5m7lda3rylirxyly9diy55lh149z-glibc-2.25-49-dev/include/bits/unistd.h:44
#2 0x0000000000471f98 nix::NixRepl::getLine(std::__cxx11::basic_string, std::allocator >&, std::__cxx11::basic_string, std::allocator > const&)
src/nix/repl.cc:187
...
```
`gdb` should also obtain debug info automatically:
```
$ gdb -p 22178
(gdb) bt
#0 0x00007fe96642436f in futex_wait_cancelable (private=, expected=0, futex_word=0x7fffa120e3f8) at ../sysdeps/unix/sysv/linux/futex-internal.h:88
#1 __pthread_cond_wait_common (abstime=0x0, mutex=0x7fffa120e300, cond=0x7fffa120e3d0) at pthread_cond_wait.c:502
#2 __pthread_cond_wait (cond=0x7fffa120e3d0, mutex=0x7fffa120e300) at pthread_cond_wait.c:655
#3 0x00007fe966c0dd1c in std::condition_variable::wait(std::unique_lock&) () from target:/nix/store/lc9cdddv2xv45ighz8znsanjfgkcdgbx-gcc-5.4.0-lib/lib/libstdc++.so.6
#4 0x00007fe96712a044 in nix::Sync::Lock::wait (cv=..., this=0x7fffa120e0c0) at src/libutil/sync.hh:56
#5 nix::ThreadPool::process (this=this@entry=0x7fffa120e2f0) at src/libutil/thread-pool.cc:60
#6 0x00007fe96747c306 in nix::Store::queryMissing (this=this@entry=0xfc57b0, targets=..., willBuild_=..., willSubstitute_=..., unknown_=..., downloadSize_=@0x7fffa120e500: 0, narSize_=@0x7fffa120e508: 0) at src/libstore/misc.cc:237
#7 0x0000000000415672 in opServe (opFlags=..., opArgs=...) at src/nix-store/nix-store.cc:836
#8 0x000000000040b632 in ::operator()(void) const (__closure=) at src/nix-store/nix-store.cc:1070
#9 0x00007fe967720642 in std::function::operator()() const (this=0x7fffa120f6f0) at /nix/store/2m6v6lnmnf9521ixd5v0x4qrmjkn30ws-gcc-5.4.0/include/c++/5.4.0/functional:2267
#10 nix::handleExceptions(std::__cxx11::basic_string, std::allocator > const&, std::function) (programName=..., fun=...) at src/libmain/shared.cc:271
#11 0x00000000004079b7 in main (argc=3, argv=0x7fffa120f818) at src/nix-store/nix-store.cc:1071
(gdb) select-frame 5
(gdb) info locals
state = {s = 0x7fffa120e300, lk = {_M_device = 0x7fffa120e300, _M_owns = true}}
(gdb) p state.s.data.active
$2 = 0
```
## Operation
ELF binaries have a unique build ID embedded in them, allowing
separate debug info files to be looked up. For example, here is the
build ID of a particular build of `libnixexpr.so`:
```
$ readelf -a /nix/store/c0srp6xfqyrjrmqhd1pgw6hcrhcghg87-nix-1.12pre5619_346aeee1/lib/libnixexpr.so | grep 'Build ID'
Build ID: bde350fa1f1bbde3649bfce3ae143b87683d8bf9
```
Given this build ID, `gdb` and `elfutils` will look in
`/run/dwarffs/.build-id/bd/e350fa1f1bbde3649bfce3ae143b87683d8bf9.debug`
to obtain the debug info for this binary. `dwarffs` will then look in
`https://cache.nixos.org/debuginfo/`:
```
$ curl https://cache.nixos.org/debuginfo/bde350fa1f1bbde3649bfce3ae143b87683d8bf9
{"archive":"../nar/0jz52hawk8wida5b2544i484mws9ddkl8jjpjmw6rzrjf83jr5zw.nar.xz","member":"lib/debug/.build-id/bd/e350fa1f1bbde3649bfce3ae143b87683d8bf9.debug"}
```
That is, the debug info is contained in the NAR file
`https://cache.nixos.org/nar/0jz52hawk8wida5b2544i484mws9ddkl8jjpjmw6rzrjf83jr5zw.nar.xz`
(along with the debug info for other binaries from the same
package). `dwarffs` will unpack all these debug info files to a cache
(`/var/cache/dwarffs/`) and serve them from there.