Date: Tue, 27 Jan 2015 15:32:27 -0800 From: Andy Lutomirski <luto@...capital.net> To: musl@...ts.openwall.com Subject: Re: Custom __set_thread_area for ARM On 01/21/2015 09:10 PM, Rich Felker wrote: > On Wed, Jan 21, 2015 at 05:45:56PM -0800, Andy Lutomirski wrote: >> On 01/13/2015 07:09 PM, Rich Felker wrote: >>> On Tue, Jan 13, 2015 at 11:19:44PM +0300, Oleg Ranevskyy wrote: >>>> Dear community, >>>> >>>> Musl has a generic implementation of the __set_thread_area function in >>>> src/thread/__set_thread_area.c. It is not used for ARM though. There is a >>>> custom ARM implementation in src/thread/arm/__set_thread_area.s. >>>> >>>> Would you be able to clarify the following question please? >>>> Why musl doesn't define SYS_set_thread_area for ARM to utilize the generic >>>> function and uses custom __set_thread_area instead? >>> >>> The ARM kernel does not implement SYS_set_thread_area. Instead it >>> provides an ARM-specific syscall. The asm file you're looking at uses >>> that instead. >>> >>> BTW, this code is replaced in git master and the pending 1.1.6 >>> release. It's part of the ARM atomics/TLS access overhaul. >> >> As the sort-of-maintainer of the kernel side of this on x86, I have to >> ask: why is the i386 __set_thread_area function written in assembly? > > First, there's the musl-technical answer: the build system's per-arch > replacement files are asm, not C. It's possible to use C by making an > empty asm file and putting the replacement C file in the arch dir, but > it's not as obvious what's going on. This could be changed if it > helped, but... > > For __set_thread_area, there are good reasons for it to be asm. The > x86 set_thread_area syscall is not usable without asm because you have > to load the resulting segment into %gs. And as for musl in particular, > we don't want an arch-specific function signature like the kernel has > for this one on x86, taking a pointer to a user_desc struct. We want > the function to simply take the desired thread pointer value and load > it. On some archs that doesn't even need a syscall; it's just loading > the argument into a GPR. On x86, however, it requires setting up a > user_desc structure, passing that to the kernel, then loading %gs > based on the result. Since we also want binaries that don't crash on > ancient (2.4) kernels (even though they can't support threads), we > also need the fallback code to use the modify_ldt syscall when > set_thread_area is not available. > > BTW you can find some documentation of the history of musl's > __set_thread_area via: > > git log -p src/thread/i386/__set_thread_area.s Does musl not use inline asm? ISTM something like: struct user_desc desc; memset(&desc, 0, sizeof(desc)); desc.base = whatever; // assign other fields if (set_thread_area(&desc) != 0) handle error; asm volatile ("mov %0,%%fs" : : "=rm" ((desc.entry_number << 3) | 3)); would be a lot more comprehensible. --Andy
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