[PATCH 3/4] kexec/kexec-zlib.h: Add 'is_zlib_file()' helper function

[Bhupesh Sharma <bhsharma@redhat.com>]

This patch adds 'is_zlib_file()' helper function which can be
used to quickly determine with the passed kernel image is a zlib
compressed kernel image.

This is specifically useful for arm64 zImage (or Image.gz) support,
which is introduced by later patches in this patchset.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 kexec/kexec-zlib.h |  1 +
 kexec/zlib.c       | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/kexec/kexec-zlib.h b/kexec/kexec-zlib.h
index 43c107bf4a72..16300f294759 100644
--- a/kexec/kexec-zlib.h
+++ b/kexec/kexec-zlib.h
@@ -6,5 +6,6 @@
 
 #include "config.h"
 
+int is_zlib_file(const char *filename, off_t *r_size);
 char *zlib_decompress_file(const char *filename, off_t *r_size);
 #endif /* __KEXEC_ZLIB_H */
diff --git a/kexec/zlib.c b/kexec/zlib.c
index 95b608059d41..34d5ca566769 100644
--- a/kexec/zlib.c
+++ b/kexec/zlib.c
@@ -23,6 +23,32 @@ static void _gzerror(gzFile fp, int *errnum, const char **errmsg)
 	}
 }
 
+int is_zlib_file(const char *filename, off_t *r_size)
+{
+	gzFile fp;
+	int errnum;
+	const char *msg;
+
+	if (!filename)
+		goto out;
+
+	fp = gzopen(filename, "rb");
+	if (fp == 0) {
+		_gzerror(fp, &errnum, &msg);
+		dbgprintf("Cannot open `%s': %s\n", filename, msg);
+		goto out;
+	}
+
+	if (gzdirect(fp))
+		/* It's not in gzip format */
+		goto out;
+
+	/* It's in gzip format */
+	return 1;
+out:
+	return 0;
+}
+
 char *zlib_decompress_file(const char *filename, off_t *r_size)
 {
 	gzFile fp;
@@ -84,6 +110,12 @@ fail:
 	return buf;
 }
 #else
+
+int is_zlib_file(const char *filename, off_t *r_size)
+{
+	return 0;
+}
+
 char *zlib_decompress_file(const char *UNUSED(filename), off_t *UNUSED(r_size))
 {
 	return NULL;
-- 
2.7.4


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