libelf: Only fetch shdr once in elf_compress[_gnu]

Some compilers assume the second call to elf[32|64]_getshdr can fail
and produce error: potential null pointer dereference. Just store the
result of the first call and reuse (when not NULL).

       * libelf/elf_compress.c (elf_compress): Store getshdr result in
       a shdr union var.
       * libelf/elf_compress_gnu.c (): Likewise

https://sourceware.org/bugzilla/show_bug.cgi?id=32311

Signed-off-by: Mark Wielaard <[email protected]>

2 files changed, 48 insertions, 52 deletions

diff --git a/libelf/elf_compress.c b/libelf/elf_compress.c
index 2cd32a6f..a2fa1f08 100644
--- a/libelf/elf_compress.c
+++ b/libelf/elf_compress.c
@@ -584,25 +584,30 @@ elf_compress (Elf_Scn *scn, int type, unsigned int flags)
   Elf64_Xword sh_flags;
   Elf64_Word sh_type;
   Elf64_Xword sh_addralign;
+  union shdr
+  {
+    Elf32_Shdr *s32;
+    Elf64_Shdr *s64;
+  } shdr;
   if (elfclass == ELFCLASS32)
     {
-      Elf32_Shdr *shdr = elf32_getshdr (scn);
-      if (shdr == NULL)
+      shdr.s32 = elf32_getshdr (scn);
+      if (shdr.s32 == NULL)
 	return -1;
 
-      sh_flags = shdr->sh_flags;
-      sh_type = shdr->sh_type;
-      sh_addralign = shdr->sh_addralign;
+      sh_flags = shdr.s32->sh_flags;
+      sh_type = shdr.s32->sh_type;
+      sh_addralign = shdr.s32->sh_addralign;
     }
   else
     {
-      Elf64_Shdr *shdr = elf64_getshdr (scn);
-      if (shdr == NULL)
+      shdr.s64 = elf64_getshdr (scn);
+      if (shdr.s64 == NULL)
 	return -1;
 
-      sh_flags = shdr->sh_flags;
-      sh_type = shdr->sh_type;
-      sh_addralign = shdr->sh_addralign;
+      sh_flags = shdr.s64->sh_flags;
+      sh_type = shdr.s64->sh_type;
+      sh_addralign = shdr.s64->sh_addralign;
     }
 
   if ((sh_flags & SHF_ALLOC) != 0)
@@ -679,17 +684,17 @@ elf_compress (Elf_Scn *scn, int type, unsigned int flags)
 	 correctly and ignored when SHF_COMPRESSED is set.  */
       if (elfclass == ELFCLASS32)
 	{
-	  Elf32_Shdr *shdr = elf32_getshdr (scn);
-	  shdr->sh_size = new_size;
-	  shdr->sh_addralign = __libelf_type_align (ELFCLASS32, ELF_T_CHDR);
-	  shdr->sh_flags |= SHF_COMPRESSED;
+	  shdr.s32->sh_size = new_size;
+	  shdr.s32->sh_addralign = __libelf_type_align (ELFCLASS32,
+							ELF_T_CHDR);
+	  shdr.s32->sh_flags |= SHF_COMPRESSED;
 	}
       else
 	{
-	  Elf64_Shdr *shdr = elf64_getshdr (scn);
-	  shdr->sh_size = new_size;
-	  shdr->sh_addralign = __libelf_type_align (ELFCLASS64, ELF_T_CHDR);
-	  shdr->sh_flags |= SHF_COMPRESSED;
+	  shdr.s64->sh_size = new_size;
+	  shdr.s64->sh_addralign = __libelf_type_align (ELFCLASS64,
+							ELF_T_CHDR);
+	  shdr.s64->sh_flags |= SHF_COMPRESSED;
 	}
 
       __libelf_reset_rawdata (scn, out_buf, new_size, 1, ELF_T_CHDR);
@@ -731,17 +736,15 @@ elf_compress (Elf_Scn *scn, int type, unsigned int flags)
 	 correctly and ignored when SHF_COMPRESSED is set.  */
       if (elfclass == ELFCLASS32)
 	{
-	  Elf32_Shdr *shdr = elf32_getshdr (scn);
-	  shdr->sh_size = scn->zdata_size;
-	  shdr->sh_addralign = scn->zdata_align;
-	  shdr->sh_flags &= ~SHF_COMPRESSED;
+	  shdr.s32->sh_size = scn->zdata_size;
+	  shdr.s32->sh_addralign = scn->zdata_align;
+	  shdr.s32->sh_flags &= ~SHF_COMPRESSED;
 	}
       else
 	{
-	  Elf64_Shdr *shdr = elf64_getshdr (scn);
-	  shdr->sh_size = scn->zdata_size;
-	  shdr->sh_addralign = scn->zdata_align;
-	  shdr->sh_flags &= ~SHF_COMPRESSED;
+	  shdr.s64->sh_size = scn->zdata_size;
+	  shdr.s64->sh_addralign = scn->zdata_align;
+	  shdr.s64->sh_flags &= ~SHF_COMPRESSED;
 	}
 
       __libelf_reset_rawdata (scn, scn->zdata_base,
diff --git a/libelf/elf_compress_gnu.c b/libelf/elf_compress_gnu.c
index 8e20b30e..006e2ae4 100644
--- a/libelf/elf_compress_gnu.c
+++ b/libelf/elf_compress_gnu.c
@@ -59,25 +59,30 @@ elf_compress_gnu (Elf_Scn *scn, int inflate, unsigned int flags)
   Elf64_Xword sh_flags;
   Elf64_Word sh_type;
   Elf64_Xword sh_addralign;
+  union shdr
+  {
+    Elf32_Shdr *s32;
+    Elf64_Shdr *s64;
+  } shdr;
   if (elfclass == ELFCLASS32)
     {
-      Elf32_Shdr *shdr = elf32_getshdr (scn);
-      if (shdr == NULL)
+      shdr.s32 = elf32_getshdr (scn);
+      if (shdr.s32 == NULL)
 	return -1;
 
-      sh_flags = shdr->sh_flags;
-      sh_type = shdr->sh_type;
-      sh_addralign = shdr->sh_addralign;
+      sh_flags = shdr.s32->sh_flags;
+      sh_type = shdr.s32->sh_type;
+      sh_addralign = shdr.s32->sh_addralign;
     }
   else
     {
-      Elf64_Shdr *shdr = elf64_getshdr (scn);
-      if (shdr == NULL)
+      shdr.s64 = elf64_getshdr (scn);
+      if (shdr.s64 == NULL)
 	return -1;
 
-      sh_flags = shdr->sh_flags;
-      sh_type = shdr->sh_type;
-      sh_addralign = shdr->sh_addralign;
+      sh_flags = shdr.s64->sh_flags;
+      sh_type = shdr.s64->sh_type;
+      sh_addralign = shdr.s64->sh_addralign;
     }
 
   /* Allocated sections, or sections that are already are compressed
@@ -122,15 +127,9 @@ elf_compress_gnu (Elf_Scn *scn, int inflate, unsigned int flags)
 	 sh_flags won't have a SHF_COMPRESSED hint in the GNU format.
 	 Just adjust the sh_size.  */
       if (elfclass == ELFCLASS32)
-	{
-	  Elf32_Shdr *shdr = elf32_getshdr (scn);
-	  shdr->sh_size = new_size;
-	}
+	  shdr.s32->sh_size = new_size;
       else
-	{
-	  Elf64_Shdr *shdr = elf64_getshdr (scn);
-	  shdr->sh_size = new_size;
-	}
+	  shdr.s64->sh_size = new_size;
 
       __libelf_reset_rawdata (scn, out_buf, new_size, 1, ELF_T_BYTE);
 
@@ -187,15 +186,9 @@ elf_compress_gnu (Elf_Scn *scn, int inflate, unsigned int flags)
 	 sh_flags won't have a SHF_COMPRESSED hint in the GNU format.
 	 Just adjust the sh_size.  */
       if (elfclass == ELFCLASS32)
-	{
-	  Elf32_Shdr *shdr = elf32_getshdr (scn);
-	  shdr->sh_size = size;
-	}
+	shdr.s32->sh_size = size;
       else
-	{
-	  Elf64_Shdr *shdr = elf64_getshdr (scn);
-	  shdr->sh_size = size;
-	}
+	shdr.s64->sh_size = size;
 
       __libelf_reset_rawdata (scn, buf_out, size, sh_addralign,
 			      __libelf_data_type (&ehdr, sh_type,