Re: [PATCH] make getcwd fail if it cannot obtain an absolute path

[Rich Felker <dalias@libc.org>]

On Fri, Jan 12, 2018 at 06:12:24PM +0300, Dmitry V. Levin wrote:
> Currently getcwd(3) can succeed without returning an absolute path
> because the underlying getcwd syscall, starting with linux commit
> v2.6.36-rc1~96^2~2, may succeed without returning an absolute path.
> 
> This is a conformance issue because "The getcwd() function shall
> place an absolute pathname of the current working directory
> in the array pointed to by buf, and return buf".
> 
> Fix this by checking the path returned by syscall and failing with
> ENOENT if the path is not absolute.  The error code is chosen for
> consistency with the case when the current directory is unlinked.
> 
> Similar issue was fixed in glibc recently, see
> https://sourceware.org/bugzilla/show_bug.cgi?id=22679
> ---
>  src/unistd/getcwd.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/src/unistd/getcwd.c b/src/unistd/getcwd.c
> index a7b925d..103fbbb 100644
> --- a/src/unistd/getcwd.c
> +++ b/src/unistd/getcwd.c
> @@ -14,6 +14,12 @@ char *getcwd(char *buf, size_t size)
>  		errno = EINVAL;
>  		return 0;
>  	}
> -	if (syscall(SYS_getcwd, buf, size) < 0) return 0;
> +	long ret = syscall(SYS_getcwd, buf, size);
> +	if (ret < 0)
> +		return 0;
> +	if (ret == 0 || buf[0] != '/') {
> +		errno = ENOENT;
> +		return 0;
> +	}
>  	return buf == tmp ? strdup(buf) : buf;
>  }
> -- 
> ldv

Looks ok. Can you provide any details on the circumstances under which
the kernel bug manifests? This would help users who may be affected
assess the severity of the situation.

Rich