Hello Alexander, On 5/16/20 12:02 PM, Alexander Monakov wrote: > Avoid implying that use of IFUNC is the only way to produce a symbol > with NULL value. Give more scenarios how a symbol may get NULL value, > but explain that in those scenarios dlsym will fail with Glibc ld.so > due to an implementation inconsistency. > > Signed-off-by: Alexander Monakov <amonakov@xxxxxxxxx> Thanks for the nicely written patch! Applied. Cheers, Michael > --- > Hello! > > Michael, in > https://stackoverflow.com/questions/13941944/why-can-the-value-of-the-symbol-returned-by-dlsym-be-null > you're in agreement with Petr Skocik that dlsym failure on NULL absolute symbols > is a Glibc bug. I'm also in agreement. > > In any case, current wording implies that apart from IFUNC there's no way a > symbol will have NULL value, which is not true: the symbol may have NULL value, > only it can't be looked up via dlsym. This patch clarifies the situation. > > Thanks. > Alexander > > > man3/dlsym.3 | 22 ++++++++++++++-------- > 1 file changed, 14 insertions(+), 8 deletions(-) > > diff --git a/man3/dlsym.3 b/man3/dlsym.3 > index 8a1ae7789..5ea151209 100644 > --- a/man3/dlsym.3 > +++ b/man3/dlsym.3 > @@ -143,16 +143,22 @@ The > .BR dlvsym () > function is a GNU extension. > .SH NOTES > -The value of a symbol returned by > +There are several scenarios when the address of a global symbol is NULL. > +For example, a symbol can be placed at zero address by the linker, via > +a linker script or with > +.I --defsym > +command-line option. Undefined weak symbols also have NULL value. > +Finally, the symbol value may be the result of > +a GNU indirect function (IFUNC) resolver function that returns > +NULL as the resolved value. In the latter case, > .BR dlsym () > -will never be NULL if the shared object is the result of normal compilation, > -since a global symbol is never placed at the NULL address. > -There are nevertheless cases where a lookup using > +also returns NULL without error. However, in the former two cases, the > +behavior of GNU dynamic linker is inconsistent: relocation processing > +succeeds and the symbol can be observed to have NULL value, but > .BR dlsym () > -may return NULL as the value of a symbol. > -For example, the symbol value may be the result of > -a GNU indirect function (IFUNC) resolver function that returns > -NULL as the resolved value. > +fails and > +.BR dlerror () > +indicates a lookup error. > .\" > .SS History > The > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
