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Fix symlink handling #9363

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merged 1 commit into from
Nov 17, 2023
Merged

Fix symlink handling #9363

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2 changes: 1 addition & 1 deletion src/libexpr/eval.hh
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Expand Up @@ -827,7 +827,7 @@ std::string showType(const Value & v);
/**
* If `path` refers to a directory, then append "/default.nix".
*/
SourcePath resolveExprPath(const SourcePath & path);
SourcePath resolveExprPath(SourcePath path);

struct InvalidPathError : EvalError
{
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18 changes: 13 additions & 5 deletions src/libexpr/parser.y
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Expand Up @@ -686,17 +686,25 @@ Expr * EvalState::parse(
}


SourcePath resolveExprPath(const SourcePath & path)
SourcePath resolveExprPath(SourcePath path)
{
unsigned int followCount = 0, maxFollow = 1024;
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Suggested change
unsigned int followCount = 0, maxFollow = 1024;
unsigned int followCount = 0, maxFollow = 1000;

1024 would suggest some base-2 exponential behavior.


/* If `path' is a symlink, follow it. This is so that relative
path references work. */
auto path2 = path.resolveSymlinks();
while (true) {
// Basic cycle/depth limit to avoid infinite loops.
if (++followCount >= maxFollow)
throw Error("too many symbolic links encountered while traversing the path '%s'", path);
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We may want to say what limit it ran into?

Suggested change
throw Error("too many symbolic links encountered while traversing the path '%s'", path);
throw Error("too many symbolic links encountered while traversing the path '%s' (limit is %d)", path, maxFollow);

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Nah, if you have more than 1024 symlinks, it's almost certainly a cycle.

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Then let's be explicit about intent, then users will know what to do with the error:

Suggested change
throw Error("too many symbolic links encountered while traversing the path '%s'", path);
throw Error("more than %d symbolic links encountered while traversing the path '%s' (there is probably a cycle)", path, maxFollow);

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coreutils:

$ ln -s bar foo
$ ln -s foo bar
$ cat foo
cat: foo: Too many levels of symbolic links

Of course coreutils is kinda bad with error messages, but we can align with it for a consistent user experience and improve on it.

Suggested change
throw Error("too many symbolic links encountered while traversing the path '%s'", path);
throw Error("too many levels of symbolic links while traversing the path '%s'; assuming it's part of a cycle after %d indirections", originalPath, maxFollows);

It would also be slightly more helpful to show the original path instead of an arbitrary path within the cycle, in case it's not the cycle that's the mistake, but rather the use of the cycle. Also note that the original symlink does not even have to be part of the cycle; it only has to lead to it.

if (path.lstat().type != InputAccessor::tSymlink) break;
path = {path.accessor, CanonPath(path.readLink(), path.path.parent().value_or(CanonPath::root))};
}
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So if I understand correctly, this is a "shallow" version of resolveSymlinks that only resolves the final path component?

In any case, this code should have a name and not be in the parser. Could you put this in a method like SourcePath::followSymlink perhaps?

I haven't had enough coffee yet to really understand the subtlety between the two methods yet, so if you could describe that in the doc comments, that will probably even help those who aren't caffeine deprived.


/* If `path' refers to a directory, append `/default.nix'. */
if (path2.lstat().type == InputAccessor::tDirectory)
return path2 + "default.nix";
if (path.lstat().type == InputAccessor::tDirectory)
return path + "default.nix";

return path2;
return path;
}


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1 change: 1 addition & 0 deletions tests/functional/lang/eval-okay-symlink-resolution.exp
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@@ -0,0 +1 @@
"test"
1 change: 1 addition & 0 deletions tests/functional/lang/eval-okay-symlink-resolution.nix
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@@ -0,0 +1 @@
import symlink-resolution/foo/overlays/overlay.nix
1 change: 1 addition & 0 deletions tests/functional/lang/symlink-resolution/foo/lib/default.nix
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"test"
1 change: 1 addition & 0 deletions tests/functional/lang/symlink-resolution/foo/overlays
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../overlays
1 change: 1 addition & 0 deletions tests/functional/lang/symlink-resolution/overlays/overlay.nix
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import ../lib