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Add support for reading symlinks longer than PATH_MAX
to readlink
and readlinkat
#1231
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Original file line number | Diff line number | Diff line change |
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@@ -180,33 +180,80 @@ pub fn renameat<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(old_dirfd: Option<Ra | |
Errno::result(res).map(drop) | ||
} | ||
|
||
fn wrap_readlink_result(v: &mut Vec<u8>, res: ssize_t) -> Result<OsString> { | ||
match Errno::result(res) { | ||
Err(err) => Err(err), | ||
Ok(len) => { | ||
unsafe { v.set_len(len as usize) } | ||
Ok(OsString::from_vec(v.to_vec())) | ||
fn wrap_readlink_result(mut v: Vec<u8>, len: ssize_t) -> Result<OsString> { | ||
unsafe { v.set_len(len as usize) } | ||
v.shrink_to_fit(); | ||
Ok(OsString::from_vec(v.to_vec())) | ||
} | ||
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fn readlink_maybe_at<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P, | ||
v: &mut Vec<u8>) | ||
-> Result<libc::ssize_t> { | ||
path.with_nix_path(|cstr| { | ||
unsafe { | ||
match dirfd { | ||
Some(dirfd) => libc::readlinkat(dirfd, cstr.as_ptr(), | ||
v.as_mut_ptr() as *mut c_char, | ||
v.capacity() as size_t), | ||
None => libc::readlink(cstr.as_ptr(), | ||
v.as_mut_ptr() as *mut c_char, | ||
v.capacity() as size_t), | ||
} | ||
} | ||
} | ||
}) | ||
} | ||
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pub fn readlink<P: ?Sized + NixPath>(path: &P) -> Result<OsString> { | ||
fn inner_readlink<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P) | ||
-> Result<OsString> { | ||
let mut v = Vec::with_capacity(libc::PATH_MAX as usize); | ||
let res = path.with_nix_path(|cstr| { | ||
unsafe { libc::readlink(cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t) } | ||
})?; | ||
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wrap_readlink_result(&mut v, res) | ||
// simple case: result is strictly less than `PATH_MAX` | ||
let res = readlink_maybe_at(dirfd, path, &mut v)?; | ||
let len = Errno::result(res)?; | ||
debug_assert!(len >= 0); | ||
if (len as usize) < v.capacity() { | ||
return wrap_readlink_result(v, res); | ||
} | ||
// Uh oh, the result is too long... | ||
// Let's try to ask lstat how many bytes to allocate. | ||
let reported_size = super::sys::stat::lstat(path) | ||
.and_then(|x| Ok(x.st_size)).unwrap_or(0); | ||
let mut try_size = if reported_size > 0 { | ||
// Note: even if `lstat`'s apparently valid answer turns out to be | ||
// wrong, we will still read the full symlink no matter what. | ||
reported_size as usize + 1 | ||
} else { | ||
// If lstat doesn't cooperate, or reports an error, be a little less | ||
// precise. | ||
(libc::PATH_MAX as usize).max(128) << 1 | ||
}; | ||
loop { | ||
v.reserve_exact(try_size); | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Interesting. I think this may be the first good use of |
||
let res = readlink_maybe_at(dirfd, path, &mut v)?; | ||
let len = Errno::result(res)?; | ||
debug_assert!(len >= 0); | ||
if (len as usize) < v.capacity() { | ||
break wrap_readlink_result(v, res); | ||
} | ||
else { | ||
// Ugh! Still not big enough! | ||
match try_size.checked_shl(1) { | ||
Some(next_size) => try_size = next_size, | ||
// It's absurd that this would happen, but handle it sanely | ||
// anyway. | ||
None => break Err(super::Error::Sys(Errno::ENAMETOOLONG)) | ||
} | ||
} | ||
} | ||
} | ||
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pub fn readlink<P: ?Sized + NixPath>(path: &P) -> Result<OsString> { | ||
inner_readlink(None, path) | ||
} | ||
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pub fn readlinkat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P) -> Result<OsString> { | ||
let mut v = Vec::with_capacity(libc::PATH_MAX as usize); | ||
let res = path.with_nix_path(|cstr| { | ||
unsafe { libc::readlinkat(dirfd, cstr.as_ptr(), v.as_mut_ptr() as *mut c_char, v.capacity() as size_t) } | ||
})?; | ||
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wrap_readlink_result(&mut v, res) | ||
pub fn readlinkat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P) | ||
-> Result<OsString> { | ||
inner_readlink(Some(dirfd), path) | ||
} | ||
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/// Computes the raw fd consumed by a function of the form `*at`. | ||
|
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You could simply this line into
.map_or(0, |x| x.st_size)