work around linux bug in readlink syscall with zero buffer size

linux fails with EINVAL when a zero buffer size is passed to the
syscall. this is non-conforming because POSIX already defines EINVAL
with a significantly different meaning: the target is not a symlink.

since the request is semantically valid, patch it up by using a dummy
buffer of length one, and truncating the return value to zero if it
succeeds.

diff --git a/src/unistd/readlink.c b/src/unistd/readlink.c
index a152d52..32f4537 100644 (file)
--- a/src/unistd/readlink.c
+++ b/src/unistd/readlink.c
@@ -4,9 +4,16 @@
 
 ssize_t readlink(const char *restrict path, char *restrict buf, size_t bufsize)
 {
+       char dummy[1];
+       if (!bufsize) {
+               buf = dummy;
+               bufsize = 1;
+       }
 #ifdef SYS_readlink
-       return syscall(SYS_readlink, path, buf, bufsize);
+       int r = __syscall(SYS_readlink, path, buf, bufsize);
 #else
-       return syscall(SYS_readlinkat, AT_FDCWD, path, buf, bufsize);
+       int r = __syscall(SYS_readlinkat, AT_FDCWD, path, buf, bufsize);
 #endif
+       if (buf == dummy && r > 0) r = 0;
+       return __syscall_ret(r);
 }

diff --git a/src/unistd/readlinkat.c b/src/unistd/readlinkat.c
index 9af45cd..f79d3d1 100644 (file)
--- a/src/unistd/readlinkat.c
+++ b/src/unistd/readlinkat.c
@@ -3,5 +3,12 @@
 
 ssize_t readlinkat(int fd, const char *restrict path, char *restrict buf, size_t bufsize)
 {
-       return syscall(SYS_readlinkat, fd, path, buf, bufsize);
+       char dummy[1];
+       if (!bufsize) {
+               buf = dummy;
+               bufsize = 1;
+       }
+       int r = __syscall(SYS_readlinkat, fd, path, buf, bufsize);
+       if (buf == dummy && r > 0) r = 0;
+       return __syscall_ret(r);
 }