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Mon, 9 Jun 2025 08:10:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id or0xJAyXRmjjBAAAD6G6ig (envelope-from ); Mon, 09 Jun 2025 08:10:52 +0000 Date: Mon, 09 Jun 2025 10:10:52 +0200 Message-ID: <87zfeh72sz.wl-tiwai@suse.de> From: Takashi Iwai To: Christophe Leroy Cc: Jaroslav Kysela , Takashi Iwai , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-sound@vger.kernel.org, Herve Codina , Mark Brown Subject: Re: [PATCH v2] ALSA: pcm: Convert multiple {get/put}_user to user_access_begin/user_access_end() In-Reply-To: References: User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Queue-Id: D75DE21190 X-Rspamd-Action: no action X-Spamd-Result: default: False [-3.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; RCPT_COUNT_SEVEN(0.00)[8]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim,suse.de:mid]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; DKIM_TRACE(0.00)[suse.de:+] On Mon, 09 Jun 2025 10:00:38 +0200, Christophe Leroy wrote: > > With user access protection (Called SMAP on x86 or KUAP on powerpc) > each and every call to get_user() or put_user() performs heavy > operations to unlock and lock kernel access to userspace. > > To avoid that, perform user accesses by blocks using > user_access_begin/user_access_end() and unsafe_get_user()/ > unsafe_put_user() and alike. > > As an exemple, before the patch the 9 calls to put_user() at the > end of snd_pcm_ioctl_sync_ptr_compat() imply the following set of > instructions about 9 times (access_ok - enable user - write - disable > user): > 0.00 : c057f858: 3d 20 7f ff lis r9,32767 > 0.29 : c057f85c: 39 5e 00 14 addi r10,r30,20 > 0.77 : c057f860: 61 29 ff fc ori r9,r9,65532 > 0.32 : c057f864: 7c 0a 48 40 cmplw r10,r9 > 0.36 : c057f868: 41 a1 fb 58 bgt c057f3c0 > 0.30 : c057f86c: 3d 20 dc 00 lis r9,-9216 > 1.95 : c057f870: 7d 3a c3 a6 mtspr 794,r9 > 0.33 : c057f874: 92 8a 00 00 stw r20,0(r10) > 0.27 : c057f878: 3d 20 de 00 lis r9,-8704 > 0.28 : c057f87c: 7d 3a c3 a6 mtspr 794,r9 > ... > > A perf profile shows that in total the 9 put_user() represent 36% of > the time spent in snd_pcm_ioctl() and about 80 instructions. > > With this patch everything is done in 13 instructions and represent > only 15% of the time spent in snd_pcm_ioctl(): > > 0.57 : c057f5dc: 3d 20 dc 00 lis r9,-9216 > 0.98 : c057f5e0: 7d 3a c3 a6 mtspr 794,r9 > 0.16 : c057f5e4: 92 7f 00 04 stw r19,4(r31) > 0.63 : c057f5e8: 93 df 00 0c stw r30,12(r31) > 0.16 : c057f5ec: 93 9f 00 10 stw r28,16(r31) > 4.95 : c057f5f0: 92 9f 00 14 stw r20,20(r31) > 0.19 : c057f5f4: 92 5f 00 18 stw r18,24(r31) > 0.49 : c057f5f8: 92 bf 00 1c stw r21,28(r31) > 0.27 : c057f5fc: 93 7f 00 20 stw r27,32(r31) > 5.88 : c057f600: 93 36 00 00 stw r25,0(r22) > 0.11 : c057f604: 93 17 00 00 stw r24,0(r23) > 0.00 : c057f608: 3d 20 de 00 lis r9,-8704 > 0.79 : c057f60c: 7d 3a c3 a6 mtspr 794,r9 > > Note that here the access_ok() in user_write_access_begin() is skipped > because the exact same verification has already been performed at the > beginning of the fonction with the call to user_read_access_begin(). > > A couple more can be converted as well but require > unsafe_copy_from_user() which is not defined on x86 and arm64, so > those are left aside for the time being and will be handled in a > separate patch. > > Signed-off-by: Christophe Leroy > --- > v2: Split out the two hunks using copy_from_user() as unsafe_copy_from_user() is not implemented on x86 and arm64 yet. Thanks for the patch. The idea looks interesting, but the implementations with unsafe_get_user() leads to very ugly goto lines, and that's too bad; it makes the code flow much more difficult to follow. I guess that, in most cases this patch tries to cover, we just use another temporary variable for compat struct, copy fields locally, then run copy_to_user() in a shot instead. Takashi