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Date: Fri, 17 Apr 2020 15:56:06 +0000
From: Pascal Cuoq <>
To: "" <>
Subject: Invalid pointer subtractions in __shlim and __shgetc


both functions `__shlim` and `__shgetc` subtract the members
named `buf` and `rpos` of the struct they manipulate.

In `__shlim`, this happens in the statement `f->shcnt = f->buf - f->rpos;`.
And in `__shgetc`, in happens inside the `shcnt` macro:

#define shcnt(f) ((f)->shcnt + ((f)->rpos - (f)->buf))

In our tests, while running `testsuite` in `libc-testsuite`,
both the `__shlim` and `__shgetc` functions are reached
with `f->buf` non-null and `f->rpos` a null pointer.

This can be made visible on execution platforms other than ours
by adding a statement at the beginning of the functions:

+      if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shlim\n");
+      if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shgetc\n");

Then if, running `libc-testsuite`, you see the following, it means that
`f->buf` was non-null and `f->rpos` was null when these points were

$ ./testsuite
fdopen test passed
fcntl test passed
fnmatch test passed
XXX Problem in __shlim
XXX Problem in __shgetc
XXX Problem in __shlim
XXX Problem in __shgetc
XXX Problem in __shlim
XXX Problem in __shgetc
XXX Problem in __shlim
XXX Problem in __shgetc
XXX Problem in __shlim
XXX Problem in __shgetc
XXX Problem in __shlim
XXX Problem in __shgetc
fscanf test passed

This has been tested on the (tag: v1.2.0) branch of git://

These pointer subtractions are undefined behavior. This is slightly worse
than computing `(char*)0-(char*)0`, which is undefined in C and defined in C++,
because compilers for both C and C++ are unlikely to exploit this one
for optimization. Subtracting between a non-null pointer and a null pointer
on the other hand is undefined behavior in both languages, and it is
plausible that doing it may someday have unexpected consequences.

I mention this because similar undefined behaviors that were extremely
unlikely to cause harm have been fixed in musl in recent months,
so that this looks like something you may want to fix too.


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