llext: relocate all symbols regardless of type

In the current implementation, the LLEXT linker will only apply
relocations targeting a given symbol if it has a specfic symbol type.
This is overzealous and causes issues on some platforms, as some symbols
that need to be relocated are skipped due to being of a "bad" type.

Ignore the symbol type when performing relocation to solve this problem,
but also add checks to ensure we don't attempt to relocate symbols with
an invalid section index. If such a relocation is found, return an error
instead of ignoring the relocation entry to ensure that it is impossible
to execute code from a (partially) unrelocated LLEXT.

Also remove all hacks added to circumvent this issue:
* qemu_cortex_r5 exclusion from test cases
* unnecessary exclusion of some flags when building with LLEXT EDK

Fixes #72832.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>

include/zephyr/llext/elf.h

@@ -245,8 +245,10 @@ struct elf64_sym {
 };
 
 #define SHN_UNDEF 0
+#define SHN_LORESERVE 0xff00
 #define SHN_ABS 0xfff1
 #define SHN_COMMON 0xfff2
+#define SHN_HIRESERVE 0xffff
 
 #define STT_NOTYPE 0
 #define STT_OBJECT 1

samples/subsys/llext/edk/app/CMakeLists.txt

@@ -1,13 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 
-# It seems llext currently doesn't support some Thumb32 relocation
-# instructions generated when building the extensions with default
-# flags. As a workaround, we remove these *unrelated* flags from the
-# default flags. This allows the extensions to be built without the
-# unsupported instructions.
-# See issue #72832 for more details.
-list(APPEND LLEXT_EDK_REMOVE_FLAGS -mcpu=cortex-r5 -mfloat-abi=hard)
-
 cmake_minimum_required(VERSION 3.20.0)
 find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
 

subsys/llext/llext.c

@@ -874,18 +874,37 @@ static int llext_link(struct llext_loader *ldr, struct llext *ext, bool do_local
 				} else {
 					LOG_INF("found symbol %s at 0x%lx", name, link_addr);
 				}
-			} else if (ELF_ST_TYPE(sym.st_info) == STT_SECTION ||
-				   ELF_ST_TYPE(sym.st_info) == STT_FUNC ||
-				   ELF_ST_TYPE(sym.st_info) == STT_OBJECT) {
-				/* Link address is relative to the start of the section */
+			} else if (sym.st_shndx == SHN_ABS) {
+				/* Absolute symbol */
+				link_addr = sym.st_value;
+			} else if ((sym.st_shndx < ldr->hdr.e_shnum) &&
+				!IN_RANGE(sym.st_shndx, SHN_LORESERVE, SHN_HIRESERVE)) {
+				/* This check rejects all relocations whose target symbol
+				 * has a section index higher than the maximum possible
+				 * in this ELF file, or belongs in the reserved range:
+				 * they will be caught by the `else` below and cause an
+				 * error to be returned. This aborts the LLEXT's loading
+				 * and prevents execution of improperly relocated code,
+				 * which is dangerous.
+				 *
+				 * Note that the unsupported SHN_COMMON section is rejected
+				 * as part of this check. Also note that SHN_ABS would be
+				 * rejected as well, but we want to handle it properly:
+				 * for this reason, this check must come AFTER handling
+				 * the case where the symbol's section index is SHN_ABS!
+				 *
+				 *
+				 * For regular symbols, the link address is obtained by
+				 * adding st_value to the start address of the section
+				 * in which the target symbol resides.
+				 */
 				link_addr = (uintptr_t)ext->mem[ldr->sect_map[sym.st_shndx]]
 					+ sym.st_value;
-
-				LOG_INF("found section symbol %s addr 0x%lx", name, link_addr);
 			} else {
-				/* Nothing to relocate here */
-				LOG_DBG("not relocated");
-				continue;
+				LOG_ERR("rela section %d, entry %d: cannot apply relocation: "
+					"target symbol has unexpected section index %d (0x%X)",
+					i, j, sym.st_shndx, sym.st_shndx);
+				return -ENOEXEC;
 			}
 
 			LOG_INF("writing relocation symbol %s type %zd sym %zd at addr 0x%lx "

tests/subsys/llext/simple/testcase.yaml

@@ -7,7 +7,6 @@ common:
     - apollo4p_evb
     - apollo4p_blue_kxr_evb
     - numaker_pfm_m487 # See #63167
-    - qemu_cortex_r5 # unsupported relocations
 
 tests:
   # While there is in practice no value in compiling subsys/llext/*.c