vDSO layer that intercepts some system call wrappers for more efficient access.
Technically the vDSO library doesn't intercept. libc chooses to use either the vDSO or the syscall. This can happen either in the wrapper itself, or through a special PLT helper where the linker asks libc to resolve the symbol to populate the GOT entry. vDSO symbols have the prefix __kernel_ or __vdso_.
That's fair, sorry for my casual language in a technically nuanced discussion. I hadn't looked at this in quite a while, but it was good to review. Thanks for the prompting.
I also double-checked the glibc and musl code to make sure I wasn't misremembering, and ended up learning about IFUNC.[1] Previously I had avoided going down the rabbit hole to understand what glibc's libc_ifunc was doing. I don't think musl uses IFUNC, at least not for clock_gettime; it seems to always link the wrapper which calls the vdso through an internally managed pointer.[2]
And now I'm wondering how safe all this indirection is. For the PLT/GOT approach I think you can disable lazy binding and force the GOT table to be read-only so exploits can't overwrite the symbol addresses. But for musl's approach it doesn't seem like you can make it's internal function pointer read-only, though maybe it's more difficult to find the address of than GOT table slots.
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Technically the vDSO library doesn't intercept. libc chooses to use either the vDSO or the syscall. This can happen either in the wrapper itself, or through a special PLT helper where the linker asks libc to resolve the symbol to populate the GOT entry. vDSO symbols have the prefix __kernel_ or __vdso_.
That's fair, sorry for my casual language in a technically nuanced discussion. I hadn't looked at this in quite a while, but it was good to review. Thanks for the prompting.
https://github.com/bminor/glibc/blob/glibc-2.41/sysdeps/unix...
https://github.com/bminor/glibc/blob/glibc-2.41/sysdeps/unix...
I also double-checked the glibc and musl code to make sure I wasn't misremembering, and ended up learning about IFUNC.[1] Previously I had avoided going down the rabbit hole to understand what glibc's libc_ifunc was doing. I don't think musl uses IFUNC, at least not for clock_gettime; it seems to always link the wrapper which calls the vdso through an internally managed pointer.[2]
And now I'm wondering how safe all this indirection is. For the PLT/GOT approach I think you can disable lazy binding and force the GOT table to be read-only so exploits can't overwrite the symbol addresses. But for musl's approach it doesn't seem like you can make it's internal function pointer read-only, though maybe it's more difficult to find the address of than GOT table slots.
[1] https://sourceware.org/glibc/wiki/GNU_IFUNC [2] https://git.musl-libc.org/cgit/musl/tree/src/time/clock_gett...