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unnamed union structure members are dangerous to expose in public headers

PATCH In review Samuel Thibault 2 participants 3 messages Mar 25, 2008

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Samuel Thibault

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#0 lore raw

PATCH 1/1 unnamed union structure members are dangerous to expose in public headers

Hello,

We have been struggling with Jean because of a bug in the headers:

#include <stdio.h>
#include <stdint.h>
#include <xen/foreign/x86_64.h>

int main(int argc, char *argv[]) {
	printf("%#lx\n", sizeof(vcpu_guest_context_x86_64_t));
	return 0;
}

will return 0x13e0 when compiled without gcc -std=c99, while the expected value is 0x1430, resulting to seg faults and all kinds of tricks. Compiling with

#include "/usr/include/xen/foreign/x86_64.h"

shows why:

/usr/include/xen/foreign/x86_64.h:88: warning: declaration does not declare anything

The special case of an unnamed union field is just a gcc extension, and thus in C99 mode it considers Xen's __DECL_REG() just as an unnamed union declaration, and does not reserve data in the structure. The patch below fixes this by telling gcc that it should use its gcc extension.

Prefix unnamed union structure fields with __extension__ to make it work properly even in e.g. C99 standard mode.

Signed-off-by Samuel Thibault <[email protected]>
xen/include/public/arch-ia64.h +5 -5
diff -r be3025f6af2e xen/include/public/arch-ia64.h
@@ -165,11 +165,11 @@ struct mapped_regs {
     unsigned long  reserved1[29];
     unsigned long  vhpi;
     unsigned long  reserved2[95];
-    union {
+    __extension__ union {
         unsigned long  vgr[16];
         unsigned long bank1_regs[16]; // bank1 regs (r16-r31) when bank0 active
     };
-    union {
+    __extension__ union {
         unsigned long  vbgr[16];
         unsigned long bank0_regs[16]; // bank0 regs (r16-r31) when bank1 active
     };

@@ -180,7 +180,7 @@ struct mapped_regs {
     unsigned long  vpsr;
     unsigned long  vpr;
     unsigned long  reserved4[76];
-    union {
+    __extension__ union {
         unsigned long  vcr[128];
         struct {
             unsigned long dcr;  // CR0

@@ -214,7 +214,7 @@ struct mapped_regs {
             unsigned long rsv6[46];
         };
     };
-    union {
+    __extension__ union {
         unsigned long  reserved5[128];
         struct {
             unsigned long precover_ifs;

@@ -608,7 +608,7 @@ struct xen_ia64_opt_feature {
 struct xen_ia64_opt_feature {
 	unsigned long cmd;		/* Which feature */
 	unsigned char on;		/* Switch feature on/off */
-	union {
+	__extension__ union {
 		struct {
 				/* The page protection bit mask of the pte.
 			 	 * This will be or'ed with the pte. */
xen/include/public/arch-x86/xen-x86_64.h +1 -1
diff -r be3025f6af2e xen/include/public/arch-x86/xen-x86_64.h
@@ -142,7 +142,7 @@ struct iret_context {
 
 #ifdef __GNUC__
 /* Anonymous union includes both 32- and 64-bit names (e.g., eax/rax). */
-#define __DECL_REG(name) union { \
+#define __DECL_REG(name) __extension__ union { \
     uint64_t r ## name, e ## name; \
     uint32_t _e ## name; \
 }
tools/include/xen-foreign/mkheader.py +1 -1
diff -r be3025f6af2e tools/include/xen-foreign/mkheader.py
@@ -38,7 +38,7 @@ inttypes["x86_64"] = {
 };
 header["x86_64"] = """
 #ifdef __GNUC__
-# define __DECL_REG(name) union { uint64_t r ## name, e ## name; }
+# define __DECL_REG(name) __extension__ union { uint64_t r ## name, e ## name; }
 # define __align8__ __attribute__((aligned (8)))
 #else
 # define __DECL_REG(name) uint64_t r ## name
#1 lore raw
Quoted 10 lines · Samuel Thibault

will return 0x13e0 when compiled without gcc -std=c99, while the expected value is 0x1430, resulting to seg faults and all kinds of tricks. Compiling with

#include "/usr/include/xen/foreign/x86_64.h"

shows why:

/usr/include/xen/foreign/x86_64.h:88: warning: declaration does not declare anything

This raises two obvious questions:
1. Why building with -std=c99?
2. Why building without -Werror?

-- Keir

author
#2 lore raw
Quoted 8 lines · Keir Fraser, le Tue 25 Mar 2008 17:33:53 +0000, a écrit :
Quoted 6 lines · Samuel Thibault

will return 0x13e0 when compiled without gcc -std=c99, while the expected value is 0x1430, resulting to seg faults and all kinds of tricks. Compiling with

#include "/usr/include/xen/foreign/x86_64.h"

Quoted 7 lines
Quoted 4 lines

shows why:

/usr/include/xen/foreign/x86_64.h:88: warning: declaration does not declare anything

This raises two obvious questions:
1. Why building with -std=c99?

Because in the wild public world, people may want to.

Quoted 1 line

2. Why building without -Werror?

That wouldn't produce an error: the warning only appears when including with "" instead of <>, so that gcc spits warnings of the header itself.

Samuel