On Thu, 2021-04-22 at 11:06 -0500, Peng Yu wrote: > On 4/22/21, Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> wrote: > > On Thu, 2021-04-22 at 10:17 -0500, Peng Yu via Gcc-help wrote: > > > Hi, > > > > > > As written on Wikipedia, PIC and PIE seems to be synonymous. > > > > > > "In computing, position-independent code[1] (PIC[1]) or > > > position-independent executable (PIE)[2] is ..." > > > > > > https://dev.gentoo.org/~vapier/crt.txt > > > > > > But according to the above page, they seem to be different. Could > > > anybody explain what is the difference between them? > > > > > > > -fPIE generates code for position-independent executable (i. e., > > "main" > > program), -fPIC generates code for shared libraries. Use -fPIE for > > libraries breaks semantic interposition (some programs require it, and > > it's also very useful in debugging). Using -fPIC for an executable > > makes the compiler *lose* some opportunities to optimize the code > > (some > > optimizations are perfectly well for main program, but breaking > > semantic > > interposition in libraries). > > > > I have some code showing the semantic interposition breakage by > > misusing > > -fPIE for a lib: > > > > https://linux.xidian.edu.cn/git/xry111/pie_vs_pic > > ### program output > running the program linked to the correct shared object: > ./exe_link_to_pic > foo in exe > foo in exe > > running the program linked to the buggy (-fPIE) shared object: > ./exe_link_to_pie > foo in exe > foo in lib > ### > > According to my understanding, it is always the foo in the lib.c be > called by bar in the same file lib.c, because when lib.c is written, > people should understand so no matter how the library is linked. > > So I'd think the -fPIE result is correct. Why do you think the -fPIC > result is correct? > It's the behavior mandated by ELF specification [1]: position- independent function should be called via PLT, and if there are multiple position-independent functions with same symbol name, the one loaded first takes precedence in PLT. In this case the main executable is loaded first. [1]: https://refspecs.linuxbase.org/elf/elf.pdf -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
