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Re: [RFC] Prevent tailcall optimizations of libdl functions

On 01/25/2017 09:12 AM, Yuri Gribov wrote:
Hi all,

This is a ping on

Some libdl functions rely on return address to figure out the calling
DSO and then use this information in computation (e.g. output of dlsym
depends on which library called it).

As reported in this
may break under tailcall optimization i.e. in cases like

  return dlsym(...);

Rich Felker suggested that adding new GCC attribute is a possible

Several functions have essentially the same issue, so this seems a reasonable approach.

Another option would be to hack around dlsym to prevent
tailcall from happening e.g.
  #define dlsym(h, name) { volatile void *sym = dlsym(h, name); return
(void *)sym; }

I'm not convinced that this prevents a tail call in all cases. I'd also caution against adding yet another mechanism to preserve the caller context.

If we cannot get the GCC attribute to work, we should move dlsym into libc_nonshared.a and use __dso_handle, like we do for other functions which require the caller context. This is not the direction I want us to move in; I prefer __builtin_return_address because it is easier to maintain, but that has the problem with tail calls.


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