[Git][ghc/ghc][wip/T25577] rts/linker/MachO: Account for internal GOT references in GOT construction
Ben Gamari (@bgamari)
gitlab at gitlab.haskell.org
Thu Feb 20 22:12:32 UTC 2025
Ben Gamari pushed to branch wip/T25577 at Glasgow Haskell Compiler / GHC
Commits:
778bf9be by Ben Gamari at 2025-02-20T17:10:43-05:00
rts/linker/MachO: Account for internal GOT references in GOT construction
Previously we failed to give GOT slots to symbols which were referred to
by GOT relocations in the same module. This lead to #25577.
Fix this by explicitly traversing relocation lists and maintaining a
`needs_got` flag for each symbol.
Fixes #25577.
- - - - -
2 changed files:
- rts/linker/MachO.c
- rts/linker/MachOTypes.h
Changes:
=====================================
rts/linker/MachO.c
=====================================
@@ -68,9 +68,10 @@ void encodeAddend(ObjectCode * oc, Section * section,
/* Global Offset Table logic */
bool isGotLoad(MachORelocationInfo * ri);
-bool needGotSlot(MachONList * symbol);
+bool needGotSlot(MachOSymbol * symbol);
bool makeGot(ObjectCode * oc);
void freeGot(ObjectCode * oc);
+static void findInternalGotRefs(ObjectCode * oc);
#endif /* aarch64_HOST_ARCH */
/*
@@ -440,6 +441,48 @@ encodeAddend(ObjectCode * oc, Section * section,
barf("unsupported relocation type: %d\n", ri->r_type);
}
+/* Note [Symbols in need of GOT entries]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * As GOT entries require memory, we ideally want to avoid reserving
+ * them for symbols where they are unnecessary. Specifically, most internal
+ * symbols will not be referenced by the GOT, even in position independent code
+ * (since you can instead use direct PC-relative addressing).
+ *
+ * However, it is nevertheless possible for internal symbols to be referenced
+ * via the GOT. Consequently, we use the following strategy to determine whether
+ * a symbol needs a GOT slot:
+ *
+ * a. all undefined external symbols are given GOT entries
+ * b. all external symbols with cross-section refrences are given GOT entries
+ * c. all internal symbols for which there are GOT relocations are given GOT
+ * entries.
+ *
+ * Failing to consider (c) lead to #25577. For this we explicitly traverse
+ * the relocations in findInternalGotRefs() looking for GOT relocations
+ * referencing internal symbols, setting the MachOSymbol.needs_got flag for
+ * each.
+ */
+
+// See Note [Symbols in need of GOT entries]
+static void
+findInternalGotRefs(ObjectCode * oc)
+{
+ for (int curSection = 0; curSection < oc->n_sections; curSection++) {
+ Section * sect = &oc->sections[curSection];
+ if (sect->info == NULL)
+ continue;
+ MachOSection * msect = sect->info->macho_section; // for access convenience
+ MachORelocationInfo * relocs = sect->info->relocation_info;
+ for(uint32_t i = 0; i < msect->nreloc; i++) {
+ MachORelocationInfo *ri = &relocs[i];
+ if (isGotLoad(ri)) {
+ MachOSymbol* symbol = &oc->info->macho_symbols[ri->r_symbolnum];
+ symbol->needs_got = true;
+ }
+ }
+ }
+}
+
bool
isGotLoad(struct relocation_info * ri) {
return ri->r_type == ARM64_RELOC_GOT_LOAD_PAGE21
@@ -448,13 +491,17 @@ isGotLoad(struct relocation_info * ri) {
/*
* Check if we need a global offset table slot for a
- * given symbol
+ * given symbol. See Note [Symbols in need of GOT entries].
*/
bool
-needGotSlot(MachONList * symbol) {
- return (symbol->n_type & N_EXT) /* is an external symbol */
- && (N_UNDF == (symbol->n_type & N_TYPE) /* and is undefined */
- || NO_SECT != symbol->n_sect); /* or is defined in a
+needGotSlot(MachOSymbol * symbol) {
+ // Does it have any internal references?
+ if (symbol->needs_got)
+ return true;
+
+ return (symbol->nlist->n_type & N_EXT) /* is an external symbol */
+ && (N_UNDF == (symbol->nlist->n_type & N_TYPE) /* and is undefined */
+ || NO_SECT != symbol->nlist->n_sect); /* or is defined in a
* different section */
}
@@ -463,7 +510,7 @@ makeGot(ObjectCode * oc) {
size_t got_slots = 0;
for(size_t i=0; i < oc->info->n_macho_symbols; i++)
- if(needGotSlot(oc->info->macho_symbols[i].nlist))
+ if(needGotSlot(&oc->info->macho_symbols[i]))
got_slots += 1;
if(got_slots > 0) {
@@ -476,7 +523,7 @@ makeGot(ObjectCode * oc) {
/* update got_addr */
size_t slot = 0;
for(size_t i=0; i < oc->info->n_macho_symbols; i++)
- if(needGotSlot(oc->info->macho_symbols[i].nlist))
+ if(needGotSlot(&oc->info->macho_symbols[i]))
oc->info->macho_symbols[i].got_addr
= ((uint8_t*)oc->info->got_start)
+ (slot++ * sizeof(void *));
@@ -1452,6 +1499,8 @@ ocGetNames_MachO(ObjectCode* oc)
}
}
#if defined(aarch64_HOST_ARCH)
+ findInternalGotRefs(oc);
+
/* Setup the global offset table
* This is for symbols that are external, and not defined here.
* So that we can load their address indirectly.
@@ -1558,7 +1607,7 @@ ocResolve_MachO(ObjectCode* oc)
/* fill the GOT table */
for(size_t i = 0; i < oc->info->n_macho_symbols; i++) {
MachOSymbol * symbol = &oc->info->macho_symbols[i];
- if(needGotSlot(symbol->nlist)) {
+ if(needGotSlot(symbol)) {
if(N_UNDF == (symbol->nlist->n_type & N_TYPE)) {
/* an undefined symbol. So we need to ensure we
* have the address.
=====================================
rts/linker/MachOTypes.h
=====================================
@@ -31,6 +31,7 @@ typedef struct _MachOSymbol {
SymbolAddr * addr; /* the final resting place of the symbol */
void * got_addr; /* address of the got slot for this symbol, if any */
MachONList * nlist; /* the nlist symbol entry */
+ bool needs_got; /* See Note [Symbols in need of GOT entries] */
} MachOSymbol;
struct ObjectCodeFormatInfo {
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/778bf9be699d0e60e560b88f11c93f54ecec8352
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/778bf9be699d0e60e560b88f11c93f54ecec8352
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