kexec: x86: struct x86_linux_param_header should be packed

I think struct x86_linux_param_header should be packed. Strange that we did not do it so far.

Without packing struct size was 3824 (decimal) on my x86_64 machine. With packing it is 3820. I think there was a padding of 4 bytes at the end. So it should be harmless.

I tried to introduce more fields and that introduced padding in the middle of structure and kexec stopped working and that's how I got to know that bootparam is not packed.

---
 include/x86/x86-linux.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: kexec-tools/include/x86/x86-linux.h
===================================================================

include/x86/x86-linux.h +1 -1

@@ -198,7 +198,7 @@ struct x86_linux_param_header {
 	struct 	edd_info eddbuf[EDDMAXNR];	/* 0xd00 */
 						/* 0xeec */
 #define COMMAND_LINE_SIZE 2048
-};
+} __attribute__ ((packed));
 
 struct x86_linux_faked_param_header {
 	struct x86_linux_param_header hdr;	/* 0x00 */

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Quoted 10 lines · Vivek Goyal

I think struct x86_linux_param_header should be packed. Strange that we did not do it so far.

Without packing struct size was 3824 (decimal) on my x86_64 machine. With packing it is 3820. I think there was a padding of 4 bytes at the end. So it should be harmless.

I tried to introduce more fields and that introduced padding in the middle of structure and kexec stopped working and that's how I got to know that bootparam is not packed.

In this case that's true and x86_linux_param_header should be packed.

One more thing is,
in include/x86/x86-linux.h, we already define PACKED macro:
#define PACKED __attribute__((packed))
But within x86-linux.h, both PACKED_and __attribute__((packed)) are used.

PACKED isn't used much time and __attribute__((packed)) is quite simple and straightforward. Maybe it's time we can remove the macro and use __attribute__((packed)) directly.

I can send another patch to address this if anyone thinks it's a good idea.

Thanks
WANG Chao

Quoted 18 lines

---
 include/x86/x86-linux.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: kexec-tools/include/x86/x86-linux.h
===================================================================

include/x86/x86-linux.h +1 -1

@@ -198,7 +198,7 @@ struct x86_linux_param_header {
 	struct 	edd_info eddbuf[EDDMAXNR];	/* 0xd00 */
 						/* 0xeec */
 #define COMMAND_LINE_SIZE 2048
-};
+} __attribute__ ((packed));
 
 struct x86_linux_faked_param_header {
 	struct x86_linux_param_header hdr;	/* 0x00 */

Quoted 4 lines

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Quoted 10 lines · Vivek Goyal

I think struct x86_linux_param_header should be packed. Strange that we did not do it so far.

Without packing struct size was 3824 (decimal) on my x86_64 machine. With packing it is 3820. I think there was a padding of 4 bytes at the end. So it should be harmless.

I tried to introduce more fields and that introduced padding in the middle of structure and kexec stopped working and that's how I got to know that bootparam is not packed.

In this case that's true and x86_linux_param_header should be packed.

One more thing is,
in include/x86/x86-linux.h, we already define PACKED macro:
#define PACKED __attribute__((packed))
But within x86-linux.h, both PACKED_and __attribute__((packed)) are used.

PACKED isn't used much time and __attribute__((packed)) is quite simple and straightforward. Maybe it's time we can remove the macro and use __attribute__((packed)) directly.

I can send another patch to address this if anyone thinks it's a good idea.