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Re: [PATCH] i386: Add _startup_sbrk and _startup_fatal [BZ #21913]
On Mon, Aug 7, 2017 at 4:47 AM, Zack Weinberg <zackw@panix.com> wrote:> On Sun, Aug 6, 2017 at 6:26 PM, H.J. Lu <hjl.tools@gmail.com> wrote:>> On Linux/x86, there are 3 ways to make a system call:>>>> 1. call *%gs:SYSINFO_OFFSET. This requires TLS initialization.>> 2. call *_dl_sysinfo. This requires relocation of _dl_sysinfo.>> 3. int $0x80. This works everywhere.>>>> When an object file is compiled with PIC, #1 is prefered since it is>> faster than #3 and doesn't require relocation of _dl_sysinfo. For>> dynamic executables, ld.so initializes TLS. However, for static>> executables, before TLS is initialized by __libc_setup_tls, #3 should>> be used for syscalls. This patch adds _startup_sbrk and _startup_fatal>> to be used in static executables before __libc_setup_tls is called. By>> default, they are defined to __sbrk and __libc_fatal, respectively. On>> x86, a special _startup_sbrk is provided and _startup_fatal is turned>> into ABORT_INSTRUCTION.>>>> Any comments?>> I have questions:>> 1) When you say "Linux/x86", do you mean specifically the old 32-bit> ABI, or does this apply to x32 and/or x64 as well?It only applies to Linux/x86 since Linux/x86-64 always use "syscall"which doesn't need TLS nor relocation.> 2) This is described as fixing a bug; please add a test case. (The bug> report describes a crash during 'make install', but that's not good> enough, it should be caught by 'make check' pre-installation.)All static tests fail when GCC is default to PIE.> 3) Couldn't we avoid adding complexity in shared code by doing startup> in a slightly different order? Specifically, why can't we ensure that> TLS is initialized, at least enough of it that "call> *%gs:SYSINFO_OFFSET" works, before we attempt to make any syscalls?To initialize TLS, we need a single syscall, sbrk. My patch provides_startup_sbrk which can be used befor TLS is initialized.-- H.J.
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