On Wed, Aug 29, 2018 at 4:02 AM, Waldemar Kozaczuk <jwkozac...@gmail.com>
wrote:
> Sometimes object files are compiled with "-z now" flag and resulting
> ELF files end up with BIND_NOW setting which requires dynamic linker
> to eagerly resolve dependent symbols. OSv satisfies this requirement
> and aborts when it finds first unresolvable symbol. This harsh behavior
> is undesired as in many cases those missing symbols may never be used.
>
> So this patch changes OSv dynamic linker by relaxing its behavior
> when handling missing symbols in BIND_NOW scenario. Instead of aborting
> it simply warns about missing symbol and leaves it unresolved to trigger
> abort when potentially referenced later. So in essence we are postponing
> abort until absolutely necessary.
>
> Fixes #993
>
> Signed-off-by: Waldemar Kozaczuk <jwkozac...@gmail.com>
> ---
> arch/aarch64/arch-elf.cc | 11 ++++++++---
> arch/x64/arch-elf.cc | 11 ++++++++---
> core/elf.cc | 22 +++++++++++++---------
> include/osv/elf.hh | 4 ++--
> 4 files changed, 31 insertions(+), 17 deletions(-)
>
> diff --git a/arch/aarch64/arch-elf.cc b/arch/aarch64/arch-elf.cc
> index fe63c59b..21324568 100644
> --- a/arch/aarch64/arch-elf.cc
> +++ b/arch/aarch64/arch-elf.cc
> @@ -58,10 +58,15 @@ bool object::arch_relocate_rela(u32 type, u32 sym,
> void *addr,
> return true;
> }
>
> -bool object::arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend)
> +bool object::arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend, bool ignore_missing)
> {
> - *static_cast<void**>(addr) = symbol(sym).relocated_addr() + addend;
> - return true;
> + auto _sym = symbol(sym, ignore_missing);
> + if (_sym.symbol) {
> + *static_cast<void**>(addr) = _sym.relocated_addr() + addend;
> + return true;
> + } else {
> + return false;
> + }
> }
>
> void object::prepare_initial_tls(void* buffer, size_t size,
> diff --git a/arch/x64/arch-elf.cc b/arch/x64/arch-elf.cc
> index 5182703a..82c60eb1 100644
> --- a/arch/x64/arch-elf.cc
> +++ b/arch/x64/arch-elf.cc
> @@ -103,10 +103,15 @@ bool object::arch_relocate_rela(u32 type, u32 sym,
> void *addr,
> return true;
> }
>
> -bool object::arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend)
> +bool object::arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend, bool ignore_missing)
> {
> - *static_cast<void**>(addr) = symbol(sym).relocated_addr();
> - return true;
> + auto _sym = symbol(sym, ignore_missing);
> + if (_sym.symbol) {
> + *static_cast<void**>(addr) = _sym.relocated_addr();
> + return true;
> + } else {
> + return false;
> + }
> }
>
> void object::prepare_initial_tls(void* buffer, size_t size,
> diff --git a/core/elf.cc b/core/elf.cc
> index 4f064cb6..071b1394 100644
> --- a/core/elf.cc
> +++ b/core/elf.cc
> @@ -549,7 +549,7 @@ static std::string demangle(const char *name) {
> return ret;
> }
>
> -symbol_module object::symbol(unsigned idx)
> +symbol_module object::symbol(unsigned idx, bool ignore_missing)
> {
> auto symtab = dynamic_ptr<Elf64_Sym>(DT_SYMTAB);
> assert(dynamic_val(DT_SYMENT) == sizeof(Elf64_Sym));
> @@ -562,8 +562,12 @@ symbol_module object::symbol(unsigned idx)
> return symbol_module(sym, this);
> }
> if (!ret.symbol) {
> - abort("%s: failed looking up symbol %s\n",
> - pathname().c_str(), demangle(name).c_str());
> + if (ignore_missing) {
> + debug("%s: failed looking up symbol %s\n",
> pathname().c_str(), demangle(name).c_str());
> + } else {
> + abort("%s: failed looking up symbol %s\n",
> pathname().c_str(), demangle(name).c_str());
> + }
> + return symbol_module(nullptr, this);
>
Is this return necessary? If we don't do it, we fallback to "return ret"
below, which is if I understand correctly, symbol_module(nullptr, _prog).
Is that not good enough?
}
> return ret;
> }
> @@ -643,13 +647,13 @@ void object::relocate_pltgot()
> void *addr = _base + p->r_offset;
> if (bind_now) {
> // If on-load binding is requested (instead of the default
> lazy
> - // binding), resolve all the PLT entries now.
> + // binding), try to resolve all the PLT entries now.
> + // If symbol cannot be resolved warn about it instead of
> aborting
> u32 sym = info >> 32;
> - if (!arch_relocate_jump_slot(sym, addr, p->r_addend)) {
> - debug_early("relocate_pltgot(): failed jump slot
> relocation\n");
> - abort();
> - }
> - } else if (original_plt) {
> + if (arch_relocate_jump_slot(sym, addr, p->r_addend, true))
> + continue;
> + }
> + if (original_plt) {
> // Restore the link to the original plt.
> // We know the JUMP_SLOT entries are in plt order, and that
> // each plt entry is 16 bytes.
> diff --git a/include/osv/elf.hh b/include/osv/elf.hh
> index d9e2acd3..d7dfb31d 100644
> --- a/include/osv/elf.hh
> +++ b/include/osv/elf.hh
> @@ -378,7 +378,7 @@ private:
> std::vector<const char*> dynamic_str_array(unsigned tag);
> Elf64_Dyn& dynamic_tag(unsigned tag);
> Elf64_Dyn* _dynamic_tag(unsigned tag);
> - symbol_module symbol(unsigned idx);
> + symbol_module symbol(unsigned idx, bool ignore_missing = false);
> symbol_module symbol_other(unsigned idx);
> Elf64_Xword symbol_tls_module(unsigned idx);
> void relocate_rela();
> @@ -428,7 +428,7 @@ protected:
> // The return value is true on success, false on failure.
> bool arch_relocate_rela(u32 type, u32 sym, void *addr,
> Elf64_Sxword addend);
> - bool arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend);
> + bool arch_relocate_jump_slot(u32 sym, void *addr, Elf64_Sxword
> addend, bool bind_now = false);
>
You forgot to update the "bind_now" name here.
size_t static_tls_end() {
> if (is_core()) {
> return 0;
> --
> 2.17.1
>
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