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Date: Fri, 30 Jan 2015 16:18:22 +0100 From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> To: Andy Lutomirski <luto@...capital.net> CC: mtk.manpages@...il.com, linux-man <linux-man@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH manpages 2/2] arch_prctl.2, set_thread_area.2, get_thread_area.2: Improve TLS documentation On 01/30/2015 03:45 PM, Andy Lutomirski wrote: > On Jan 30, 2015 8:59 AM, "Michael Kerrisk (man-pages)" > <mtk.manpages@...il.com> wrote: >> >> Hi Andy, >> >> Thanks for this! >> >> I've merged this into a branch and done some light editing. >> A few questions below. >> >> On 01/29/2015 10:47 PM, Andy Lutomirski wrote: >>> The documentation for set_thread_area was very vague. This improves >>> it, accounts for recent kernel changes, and merges it with >>> get_thread_area.2. >> >> (Yep, merging seems a good idea.) >>> >>> While I'm at it, clarify the related arch_prctl.2 man page. >>> >>> Signed-off-by: Andy Lutomirski <luto@...capital.net> >>> --- >>> man2/arch_prctl.2 | 22 +++++++----- >>> man2/get_thread_area.2 | 58 +------------------------------ >>> man2/set_thread_area.2 | 93 ++++++++++++++++++++++++++++++++++++++++++++++---- >>> 3 files changed, 102 insertions(+), 71 deletions(-) >>> >>> diff --git a/man2/arch_prctl.2 b/man2/arch_prctl.2 >>> index a3ab2b566915..c89fa3c7b744 100644 >>> --- a/man2/arch_prctl.2 >>> +++ b/man2/arch_prctl.2 >>> @@ -114,27 +114,33 @@ The 64-bit base changes when a new 32-bit segment selector is loaded. >>> is disabled in some kernels. >>> >>> Context switches for 64-bit segment bases are rather expensive. >>> -It may be a faster alternative to set a 32-bit base using a >>> -segment selector by setting up an LDT with >>> -.BR modify_ldt (2) >>> -or using the >>> +As an optimization, if a 32-bit TLS base address is used, >>> +.BR arch_prctl (2) >>> +may use a real TLS entry as if >>> .BR set_thread_area (2) >>> -system call in kernel 2.5 or later. >>> -.BR arch_prctl () >>> -is needed only when you want to set bases that are larger than 4GB. >>> +had been called instead of manipulating the segment base register directly. >>> Memory in the first 2GB of address space can be allocated by using >>> .BR mmap (2) >>> with the >>> .B MAP_32BIT >>> flag. >>> >>> +As a result, using >> >> Something doesn't quite flow here. "As a result" *of what*? Should this rather >> be something like: >> >> Because of the aforementioned optimization, using > > Sure. Changed. >>> +.BR arch_prctl (2) >>> +and >>> +.BR set_thread_area (2) >>> +in the same thread is dangerous, as they may overwrite each other's >>> +TLS entries. >>> + >>> As of version 2.7, glibc provides no prototype for >>> .BR arch_prctl (). >>> You have to declare it yourself for now. >>> This may be fixed in future glibc versions. >>> >>> .I FS >>> -may be already used by the threading library. >>> +may be already used by the threading library. Glibc programs that use >> >> What is a "Glibc program"? > > A program that links against glibc. Maybe that should just say "Programs". Okay /S/Glibc programs/Programs/ > The idea I'm trying to get across is that using ARCH_SET_FS for > private purposes is a bad idea, since your threading library > (glibc/libpthread) is very likely to use ARCH_SET_FS for its own > purposes. ARCH_SET_GS, on the other hand, is more likely to be > available for private use. > > This is most relevant for programs written for the purpose of testing > arch_prctl -- using ARCH_SET_FS following by calling any glibc > function is very likely to segfault. (Okay.) [...] >>> diff --git a/man2/get_thread_area.2 b/man2/get_thread_area.2 >>> index 08589e413cc4..a03fe54fb08e 100644 >>> --- a/man2/set_thread_area.2 >>> +++ b/man2/set_thread_area.2 >>> .IR Note : >>> There is no glibc wrapper for this system call; see NOTES. >>> .SH DESCRIPTION >>> +Linux dedicates three global descriptor table (GDT) entries for >>> +thread-local storage. For more information about the LDT, see the >> >> s/LDT/GDT? in the previous line, right? > > Yes. Fixed. >>> +Intel Software Developer's Manual or the AMD Architecture Programming Manual. >>> + >>> +.BR get_thread_area () >>> +reads the GDT entry indicated by >>> +.I u_info\->entry_number >>> +and fills in the rest of the fields in >>> +.I u_info. >>> + >>> .BR set_thread_area () >>> -sets an entry in the current thread's thread-local storage (TLS) array. >>> +sets a TLS entry in the GDT. >>> +.PP >>> The TLS array entry set by >>> .BR set_thread_area () >>> corresponds to the value of >>> @@ -27,7 +56,7 @@ corresponds to the value of >>> passed in by the user. >>> If this value is in bounds, >>> .BR set_thread_area () >>> -copies the TLS descriptor pointed to by >>> +writes the TLS descriptor pointed to by >>> .I u_info >>> into the thread's TLS array. >>> .PP >>> @@ -35,12 +64,29 @@ When >>> .BR set_thread_area () >>> is passed an >>> .I entry_number >>> -of \-1, it uses a free TLS entry. >>> +of \-1, it finds a free TLS entry. >> >> s/finds/searched for/ ? > > How about "searches for"? D'oh! That's what I meant. Changed. [...] >>> +.SH BUGS >>> +On 64-bit kernels before Linux 3.19, one of the padding bits in >> >> Was that commit e30ab185c490e9a9381385529e0fd32f0a399495 ? > > Yes. Okay. Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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