lib/sys/module: Add support for weak symbols

We need a means of allowing Syslinux derivative-specific functions to
remain unresolved when there is no implementation for them. For
example, unload_pxe() is referenced by common code in com32/lib, but
is only implemented for PXELINUX. By marking unload_pxe() as __weak we
can substitute references with the undefined_symbol() function for
SYSLINUX, EXTLINUX and ISOLINUX. Provided that those references are
never executed everything works fine. However, if code calls actually
calls unload_pxe() from SYSLINUX, EXTLINUX or ISOLINUX an error
message is displayed and execution is halted.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

diff --git a/com32/lib/sys/module/common.c b/com32/lib/sys/module/common.c
index 19742e6..002b733 100644 (file)
--- a/com32/lib/sys/module/common.c
+++ b/com32/lib/sys/module/common.c
@@ -321,7 +321,7 @@ int check_symbols(struct elf_module *module)
                crt_name = module->str_table + crt_sym->st_name;
 
                strong_count = 0;
-               weak_count = 0;
+               weak_count = (ELF32_ST_BIND(crt_sym->st_info) == STB_WEAK);
 
                for_each_module(crt_module)
                {
@@ -345,6 +345,14 @@ int check_symbols(struct elf_module *module)
                if (crt_sym->st_shndx == SHN_UNDEF)
                {
                        // We have an undefined symbol
+                       //
+                       // We use the weak_count to differentiate
+                       // between Syslinux-derivative-specific
+                       // functions. For example, unload_pxe() is
+                       // only provided by PXELINUX, so we mark it as
+                       // __weak and replace it with a reference to
+                       // undefined_symbol() on SYSLINUX, EXTLINUX,
+                       // and ISOLINUX. See perform_relocations().
                        if (strong_count == 0 && weak_count == 0)
                        {
                                DBG_PRINT("Symbol %s is undefined\n", crt_name);

diff --git a/com32/lib/sys/module/elf_module.c b/com32/lib/sys/module/elf_module.c
index dbb5afe..b220e1a 100644 (file)
--- a/com32/lib/sys/module/elf_module.c
+++ b/com32/lib/sys/module/elf_module.c
@@ -11,6 +11,7 @@
 #include <stdio.h>
 #include <elf.h>
 #include <dprintf.h>
+#include <core.h>
 
 #include <linux/list.h>
 #include <sys/module.h>
@@ -238,6 +239,11 @@ static int prepare_dynlinking(struct elf_module *module) {
        return 0;
 }
 
+void undefined_symbol(void)
+{
+       printf("Error: An undefined symbol was referenced\n");
+       kaboom();
+}
 
 static int perform_relocation(struct elf_module *module, Elf32_Rel *rel) {
        Elf32_Word *dest = module_get_absolute(rel->r_offset, module);
@@ -263,11 +269,16 @@ static int perform_relocation(struct elf_module *module, Elf32_Rel *rel) {
                                        &sym_module);
 
                if (sym_def == NULL) {
-                       // This should never happen
                        DBG_PRINT("Cannot perform relocation for symbol %s\n",
                                        module->str_table + sym_ref->st_name);
 
-                       return -1;
+                       if (ELF32_ST_BIND(sym_ref->st_info) != STB_WEAK)
+                               return -1;
+
+                       // This must be a derivative-specific
+                       // function. We're OK as long as we never
+                       // execute the function.
+                       sym_def = global_find_symbol("undefined_symbol", &sym_module);
                }
 
                // Compute the absolute symbol virtual address