fs: avoid input buffering with small limits in fs.readfile()

If the read limit passed to fs.readfile() is smaller than BUFSIZ then
disable stdio input buffering to avoid overreading the underlying file.

This is useful when reading small amounts of data from special files
such as /dev/urandom, where a readfile call limited to 16 bytes might
actually read 4096 due to stdio buffering.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/fs.c b/lib/fs.c
index 30e5540..b831aee 100644
--- a/lib/fs.c
+++ b/lib/fs.c
@@ -1168,6 +1168,9 @@ uc_fs_readfile(uc_vm_t *vm, size_t nargs)
 
 	buf = ucv_stringbuf_new();
 
+	if (limit > -1 && limit < BUFSIZ)
+		setvbuf(fp, NULL, _IONBF, 0);
+
 	while (limit != 0) {
 		blen = 1024;