[PATCH] efi_loader: loosen buffer parameter check in efi_file_read

[Stefan Sørensen]

When reading a directory, EFI_BUFFER_TOO_SMALL should be returned when
the supplied buffer is too small, so a use-case is to call
efi_file_read with *buffer_size=0 and buffer=NULL to obtain the needed
size before doing the actual read.

So drop the buffer!=NULL requirement when doing directory reads, but
keep them when doing file reads.

This fix allows the Redhat shim fallback to run and e.g. Fedora 32 now
boots out of the box.

Signed-off-by: Stefan Sørensen <stefan.sorensen at spectralink.com>
---
 lib/efi_loader/efi_file.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/efi_loader/efi_file.c b/lib/efi_loader/efi_file.c
index 19afa69f53..7b0dcbdba9 100644
--- a/lib/efi_loader/efi_file.c
+++ b/lib/efi_loader/efi_file.c
@@ -349,6 +349,11 @@ static efi_status_t file_read(struct file_handle *fh, u64 *buffer_size,
 	efi_status_t ret;
 	loff_t file_size;
 
+	if (!buffer_size || !buffer) {
+		ret = EFI_INVALID_PARAMETER;
+		return ret;
+	}
+
 	ret = efi_get_file_size(fh, &file_size);
 	if (ret != EFI_SUCCESS)
 		return ret;
@@ -377,6 +382,9 @@ static efi_status_t dir_read(struct file_handle *fh, u64 *buffer_size,
 	u64 required_size;
 	u16 *dst;
 
+	if (!buffer_size || (*buffer_size && !buffer))
+		return EFI_INVALID_PARAMETER;
+
 	if (set_blk_dev(fh))
 		return EFI_DEVICE_ERROR;
 
@@ -443,7 +451,7 @@ static efi_status_t EFIAPI efi_file_read(struct efi_file_handle *file,
 
 	EFI_ENTRY("%p, %p, %p", file, buffer_size, buffer);
 
-	if (!buffer_size || !buffer) {
+	if (!buffer_size || (*buffer_size && !buffer)) {
 		ret = EFI_INVALID_PARAMETER;
 		goto error;
 	}
-- 
2.25.4