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this issue caused the address of functions in shared libraries to resolve to their PLT thunks in the main program rather than their correct addresses. it was observed causing crashes, though the mechanism of the crash was not thoroughly investigated. since the issue is very subtle, it calls for some explanation: on all well-behaved archs, GOT entries that belong to the PLT use a special relocation type, typically called JMP_SLOT, so that the dynamic linker can avoid having the jump destinations for the PLT resolve to PLT thunks themselves (they also provide a definition for the symbol, which must be used whenever the address of the function is taken so that all DSOs see the same address). however, the traditional mips PIC ABI lacked such a JMP_SLOT relocation type, presumably because, due to the way PIC works, the address of the PLT thunk was never needed and could always be ignored. prior to commitadf94c1966, the mips version of reloc.h contained a hack that caused all symbol lookups to be treated like JMP_SLOT, inhibiting undefined symbols from ever being used to resolve symbolic relocations. this hack goes all the way back to commitbabf820180, when the mips dynamic linker was first made usable. during the recent refactoring to eliminate arch-specific relocation processing (commitadf94c1966), this hack was overlooked and no equivalent functionality was provided in the new code. fixing the problem is not as simple as adding back an equivalent hack, since there is now also a "non-PIC ABI" that can be used for the main executable, which actually does use a PLT. the closest thing to official documentation I could find for this ABI is nonpic.txt, attached to Message-ID: 20080701202236.GA1534@caradoc.them.org, which can be found in the gcc mailing list archives and elsewhere. per this document, undefined symbols corresponding to PLT thunks have the STO_MIPS_PLT bit set in the symbol's st_other field. thus, I have added an arch-specific rule for mips, applied at the find_sym level rather than the relocation level, to reject undefined symbols with the STO_MIPS_PLT bit clear. the previous hack of treating all mips relocations as JMP_SLOT-like, rather than rejecting the unwanted symbols in find_sym, probably also caused dlsym to wrongly return PLT thunks in place of the correct address of a function under at least some conditions. this should now be fixed, at least for global-scope symbol lookups.
90 lines
2.1 KiB
C
90 lines
2.1 KiB
C
#include <string.h>
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#include <elf.h>
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#include <endian.h>
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#if __BYTE_ORDER == __LITTLE_ENDIAN
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#define ENDIAN_SUFFIX "el"
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#else
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#define ENDIAN_SUFFIX ""
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#endif
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#ifdef __mips_soft_float
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#define FP_SUFFIX "-sf"
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#else
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#define FP_SUFFIX ""
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#endif
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#define LDSO_ARCH "mips" ENDIAN_SUFFIX FP_SUFFIX
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#define TPOFF_K (-0x7000)
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static int remap_rel(int type)
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{
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switch(type) {
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case R_MIPS_REL32:
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return REL_SYM_OR_REL;
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case R_MIPS_JUMP_SLOT:
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return REL_PLT;
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case R_MIPS_COPY:
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return REL_COPY;
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case R_MIPS_TLS_DTPMOD32:
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return REL_DTPMOD;
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case R_MIPS_TLS_DTPREL32:
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return REL_DTPOFF;
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case R_MIPS_TLS_TPREL32:
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return REL_TPOFF;
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}
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return 0;
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}
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void __reloc_self(int c, size_t *a, size_t *dynv, size_t *got)
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{
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char *base;
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size_t t[20], n;
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for (a+=c+1; *a; a++);
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for (a++; *a; a+=2) if (*a<20) t[*a] = a[1];
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base = (char *)t[AT_BASE];
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if (!base) base = (char *)(t[AT_PHDR] & -t[AT_PAGESZ]);
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for (a=dynv; *a; a+=2) if (*a-0x70000000UL<20) t[*a&31] = a[1];
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n = t[DT_MIPS_LOCAL_GOTNO - 0x70000000];
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for (a=got; n; a++, n--) *a += (size_t)base;
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}
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static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stride);
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static void do_arch_relocs(struct dso *this, struct dso *head)
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{
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unsigned char *base = this->base;
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size_t *dynv = this->dynv;
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size_t dyn[20] = {0};
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size_t i;
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size_t rel[2], got=0;
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Sym *sym;
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for (i=0; dynv[i]; i+=2) {
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if (dynv[i]-0x70000000UL<20)
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dyn[dynv[i]&31] = dynv[i+1];
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else if (dynv[i] == DT_PLTGOT)
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got = dynv[i+1];
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}
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i = dyn[DT_MIPS_LOCAL_GOTNO-0x70000000];
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if (this->shortname && !strcmp(this->shortname, "libc.so")) {
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got += sizeof(size_t) * i;
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} else {
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for (; i; i--, got+=sizeof(size_t))
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*(size_t *)(base+got) += (size_t)base;
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}
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sym = this->syms + dyn[DT_MIPS_GOTSYM-0x70000000];
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i = dyn[DT_MIPS_SYMTABNO-0x70000000] - dyn[DT_MIPS_GOTSYM-0x70000000];
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for (; i; i--, got+=sizeof(size_t), sym++) {
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rel[0] = got;
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rel[1] = sym-this->syms << 8 | R_MIPS_JUMP_SLOT;
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*(size_t *)(base+got) = 0;
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do_relocs(this, rel, sizeof rel, 2);
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}
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}
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#define NEED_ARCH_RELOCS 1
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#define DYNAMIC_IS_RO 1
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#define ARCH_SYM_REJECT_UND(s) (!((s)->st_other & STO_MIPS_PLT))
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