From: Richard Henderson <rth@twiddle.net>
Date: Thu, 17 Apr 2025 20:58:35 +0000 (-0300)
Subject: elf: Adjust DT_EXTRATAGIDX to avoid undefined shifts
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=73c7242778e23cef2712756ba1e5aaf141de7851;p=thirdparty%2Fglibc.git

elf: Adjust DT_EXTRATAGIDX to avoid undefined shifts

When building with --enable-ubsan, the relocation code triggers:

UBSAN: Undefined behaviour in get-dynamic-info.h:56:30 left shift of 1879047925 by 1 cannot be represented in type 'int'

Originally from
https://sourceware.org/pipermail/libc-alpha/2015-August/063015.html.
---

diff --git a/elf/elf.h b/elf/elf.h
index 1e1a59c14d..d48cf47b9a 100644
--- a/elf/elf.h
+++ b/elf/elf.h
@@ -995,7 +995,7 @@ typedef struct
    range.  Be compatible.  */
 #define DT_AUXILIARY    0x7ffffffd      /* Shared object to load before self */
 #define DT_FILTER       0x7fffffff      /* Shared object to get values from */
-#define DT_EXTRATAGIDX(tag)	((Elf32_Word)-((Elf32_Sword) (tag) <<1>>1)-1)
+#define DT_EXTRATAGIDX(tag)  (-((Elf32_Sword)((Elf32_Word)(tag) * 2) / 2 + 1))
 #define DT_EXTRANUM	3
 
 /* Values of `d_un.d_val' in the DT_FLAGS entry.  */